Thermal control for formation and processing of aluminum nitride

ABSTRACT

In various embodiments, controlled heating and/or cooling conditions are utilized during the fabrication of aluminum nitride single crystals and aluminum nitride bulk polycrystalline ceramics. Thermal treatments may also be utilized to control properties of aluminum nitride crystals after fabrication.

RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/584,214, filed Nov. 10, 2017, the entiredisclosure of which is hereby incorporated herein by reference.

TECHNICAL FIELD

In various embodiments, the present invention relates to the fabricationof single-crystal aluminum nitride (AlN).

BACKGROUND

Aluminum nitride (AlN) holds great promise as a semiconductor materialfor numerous applications, e.g., optoelectronic devices such asshort-wavelength light-emitting diodes (LEDs) and lasers, dielectriclayers in optical storage media, electronic substrates, and chipcarriers where high thermal conductivity is essential, among manyothers. In principle, the properties of AlN may allow light emission atwavelengths down to around 200 nanometers (nm) to be achieved. Recentwork has demonstrated that ultraviolet (UV) LEDs have superiorperformance when fabricated on low-defect AlN substrates prepared frombulk AlN single crystals. The use of AlN substrates is also expected toimprove high-power radio-frequency (RF) devices made with nitridesemiconductors due to the high thermal conductivity with low electricalconductivity. However, the commercial feasibility of AlN-basedsemiconductor devices is limited by the scarcity and high cost of large,low-defect single crystals of AlN.

To make large-diameter AlN substrates more readily available andcost-effective, and to make the devices built thereon commerciallyfeasible, it is desirable to grow large-diameter (>25 mm) AlN bulkcrystals at a high growth rate (>0.5 mm/hr) while preserving crystalquality. The most effective method of growing AlN bulk single crystalsis the “sublimation-recondensation” method that involves sublimation oflower-quality (typically polycrystalline) AlN source material andrecondensation of the resulting vapor to form the single-crystal AlN.U.S. Pat. No. 6,770,135 (the '135 patent), U.S. Pat. No. 7,638,346 (the'346 patent), U.S. Pat. No. 7,776,153 (the '153 patent), and U.S. Pat.No. 9,028,612 (the '612 patent), the entire disclosures of which areincorporated by reference herein, describe various aspects ofsublimation-recondensation growth of AlN, both seeded and unseeded.

While AlN substrates are enabling platforms for the fabrication of UVlight-emitting devices such as LEDs, their performance in suchapplications is often limited by their transparency to UV light (i.e.,“UV transparency”) or lack thereof. AlN substrates with high UVtransparency are often difficult to produce, as UV transparency iscompromised by contamination and/or point defects introduced during theAlN growth process. Such issues have been addressed on a limited basisvia techniques disclosed in U.S. Pat. Nos. 8,012,257, 9,034,103, and9,447,519, the entire disclosure of each of which is incorporated hereinby reference. Specifically, these patents disclose techniques forcontrolling the introduction of oxygen impurities during polycrystallineAlN source-material preparation and sublimation-recondensation growth ofsingle-crystal AlN. While such techniques were reported as enablingproduction of bulk AlN crystals having low absorption coefficients, andthus high UV transparency, it has been found by the present inventorsthat such techniques are incapable of producing high UV transparencywhen utilized in conjunction with seeded growth of AlN bulk crystalsexceeding 25 mm in diameter (e.g., crystals between 30 mm and 75 mm indiameter, for example approximately 50 mm in diameter) at high growthrates (e.g., at least 0.5-0.8 mm/hr) and utilizing the large axial andradial thermal gradients necessary for such growth.

For example, FIG. 6 of U.S. Pat. No. 9,447,519 (the '519 patent) depictsan absorption spectrum of an AlN crystal produced utilizing theoxygen-control techniques and controlled post-growth cooling describedin the '519 patent. As the figure indicates, that AlN crystal had anabsorption coefficient below about 10 cm⁻¹ for the wavelength rangebetween 300 nm and 350 nm. However, that crystal was produced utilizingunseeded growth and had a maximum diameter of less than 25 mm. Thepresent inventors attempted to reproduce this high UV transparencyutilizing the same techniques for an otherwise substantially identicalgrowth process involving seeded growth of a crystal of approximately 50mm in diameter. Unfortunately, even utilizing the techniques of the '519patent, the resulting crystals were effectively opaque, i.e., exhibitingan absorption coefficient greater than 100-200 cm⁻¹ over one or more UVwavelengths, and/or exhibiting large peaks in the absorption coefficientat approximately 265 nm and/or 310 nm. An example graph of theabsorption coefficients of different wafers sliced from a boule producedusing the techniques of the '519 patent and having a diameter of 50 mmis shown in FIG. 1A. As shown, each of the wafers from this boule isessentially opaque at various UV wavelengths. The UV transparency ofsimilar crystals was particularly poor for crystals produced utilizingan on-axis AlN seed. On-axis growth is preferred for economic reasons;off-axis boules of AlN must be cut at an angle to produce on-axissubstrates therefrom, and thus the number of substrates that may beproduced from an off-axis boule is necessarily less than that which maybe produced from an on-axis boule. In view of these results, the presentinventors recognized a need for novel and improved techniques ofachieving high levels of UV transparency in larger AlN bulk crystals.

SUMMARY

In various embodiments of the present invention, growth of large,high-quality, highly UV-transparent single crystals of AlN is enabledvia techniques that limit the introduction of contaminants and defectsthat compromise UV transparency. Such techniques are particularlybeneficial for the fabrication of AlN bulk crystals having diametersgreater than 25 mm (e.g., approximately 50 mm or greater) utilizingseeded growth procedures at high growth rates (e.g., growth rates of atleast 0.5 mm/hr)—a substrate-fabrication regime in which high levels ofUV transparency and substrate quality are not achievable utilizingconventional techniques. Embodiments of the invention also enable thegrowth of large single crystals of AlN that possess both high levels ofUV transparency and high levels of crystalline quality (e.g., lowdensities of crystalline defects such as dislocations), even at crystaldiameters greater than 25 mm or of approximately 50 mm or greater.

The present inventors have discovered that the presence of carbonimpurities can lead to high levels of UV absorption in AlN bulkcrystals. Carbon incorporation leads to UV absorption at wavelengthsaround 265 nm, which can hinder the performance of UV light-emittingdevices. (In the '519 patent, control of carbon incorporation was notexplicitly contemplated, as carbon was suggested as both a possibledopant and crucible material for AlN fabrication.) In contrast, oxygenimpurities (or related point defects) typically result in UV absorptionat wavelengths around 310 nm. Thus, while control of oxygencontamination is desirable for UV transparency, it is not sufficient toenable UV transparency at many UV wavelengths. Embodiments of thepresent invention include techniques for the control and limitation ofboth carbon and oxygen contamination during an AlN growth procedure viaa physical vapor transport technique such as sublimation-recondensation.Such techniques include reduction or substantial elimination of carbonand oxygen impurities from the polycrystalline AlN source material,control of various growth parameters (e.g., growth pressure) duringfabrication, and active control of the cool-down of the grown AlNcrystal from the elevated growth temperature to reduce contaminationwhile also avoiding cracking of the material.

In various embodiments of the present invention, the radial and/or axialthermal gradients within the crystal-growth crucible utilized to promoteand control the growth of the AlN material may be controlled in variousdifferent manners. For example, individual heating elements arrangedaround the crucible may be powered to different levels (and thusdifferent temperatures) to establish thermal gradients within thecrucible. In addition or instead, thermal insulation may be selectivelyarranged around the crucible such that thinner and/or less insulatinginsulation is positioned around areas of higher desired temperature. Asdetailed in the '612 patent, thermal shields may also be arranged aroundthe crucible, e.g., above and/or below the crucible, in any of amultitude of different arrangements in order to establish desiredthermal gradients within the crucible.

Although embodiments of the invention have been presented hereinutilizing AlN as the exemplary crystalline material fabricated inaccordance therewith, embodiments of the invention may also be appliedto other crystalline materials such as silicon carbide (SiC) and zincoxide (ZnO); thus, herein, all references to AlN herein may be replaced,in other embodiments, by SiC or ZnO. As utilized herein, the term“diameter” refers to a lateral dimension (e.g., the largest lateraldimension) of a crystal, growth chamber, or other object, even if thecrystal, growth chamber, or other object is not circular and/or isirregular in cross-section.

As utilized herein, a “substrate” or a “wafer” is a portion of apreviously grown crystalline boule having top and bottom opposed,generally parallel surfaces. Substrates typically have thicknessesranging between 200 μm and 1 mm and may be utilized as platforms for theepitaxial growth of semiconductor layers and the fabrication ofsemiconductor devices (e.g., light-emitting devices such as lasers andlight-emitting diodes, transistors, power devices, etc.) thereon. Asutilized herein, “room temperature” is 25° C.

In an aspect, embodiments of the invention feature a method of formingsingle-crystal aluminum nitride (AlN). A bulk polycrystalline AlNceramic is provided. At least a portion of the AlN ceramic is disposedinto a first crucible. The at least a portion of the AlN ceramic isannealed and densified in the first crucible, thereby forming apolycrystalline AlN source. The annealing and densifying includes,consists essentially of, or consists of (i) heating the at least aportion of the AlN ceramic at a first temperature ranging from 1100° C.to 1900° C. for a first time ranging from 2 hours to 25 hours, and (ii)thereafter, heating the at least a portion of the AlN ceramic at asecond temperature ranging from 1900° C. to 2250° C. for a second timeranging from 3 hours to 15 hours, or (i) heating the at least a portionof the AlN ceramic during a temperature ramp to a third temperatureranging from 1900° C. to 2250° C. over a third time ranging from 5 hoursto 25 hours, and (ii) thereafter, heating the at least a portion of theAlN ceramic at a fourth temperature ranging from 1900° C. to 2250° C.for a fourth time ranging from 3 hours to 25 hours. The AlN source iscooled to approximately room temperature. A second crucible is disposedwithin a furnace. The second crucible contains the AlN source and a seedcrystal that includes, consists essentially of, or consists ofsingle-crystal AlN. The second crucible is heated with the furnace to agrowth temperature of at least 2000° C. The second crucible ismaintained at the growth temperature for a soak time ranging from 1 hourto 10 hours. After the soak time, while the second crucible is at thegrowth temperature, (i) vapor including, consisting essentially of, orconsisting of aluminum and nitrogen is condensed on the seed crystal,thereby forming a single-crystalline AlN boule extending from the seedcrystal, and (ii) the second crucible is moved relative to the furnace.The growth rate of at least a portion of the AlN boule is approximatelyequal to a rate of relative motion between the second crucible and thefurnace. Thereafter, the AlN boule is cooled via a cooling cycle. Thecooling cycle includes, consists essentially of, or consists of (i)cooling the AlN boule from the growth temperature to a fifth temperatureranging from 1450° C. to 2150° C. over a fifth time ranging from 10minutes to 90 minutes, and (ii) thereafter, cooling the AlN boule fromthe fifth temperature to a sixth temperature ranging from 1000° C. to1650° C. over a sixth time ranging from 10 seconds to 10 minutes, or (i)cooling the AlN boule from the growth temperature to a seventhtemperature ranging from 1450° C. to 2150° C. over a seventh timeranging from 10 seconds to 10 minutes, and (ii) thereafter, cooling theAlN boule from the seventh temperature to an eighth temperature rangingfrom 1000° C. to 1650° C. over an eighth time ranging from 10 minutes to90 minutes. Thereafter, the AlN boule is cooled to approximately roomtemperature.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The growth temperature may beapproximately 2300° C. or less, approximately 2200° C. or less, orapproximately 2100° C. or less. The diameter of at least a portion (oreven all) of the AlN boule may be at least approximately 35 mm, at leastapproximately 50 mm, at least approximately 75 mm, or at leastapproximately 100 mm. The diameter of at least a portion (or even all)of the AlN boule may be at most approximately 150 mm, at mostapproximately 125 mm, at most approximately 100 mm, or at mostapproximately 75 mm. The diameter of at least a portion (or even all) ofthe AlN boule may be approximately 50 mm. Oxygen (e.g., oxygen gas or anoxygen-containing gas) may be introduced into the second crucible duringformation of the single-crystalline AlN boule extending from the seedcrystal. Prior to disposing the at least a portion of the AlN ceramicinto the first crucible, the AlN ceramic may be fragmented intofragments. One or more (or even each) of the fragments may have a width(or diameter, or length, or other dimension) greater than approximately0.1 cm, greater than approximately 0.2 cm, greater than approximately0.3 cm, greater than approximately 0.4 cm, greater than approximately0.5 cm, greater than approximately 0.7 cm, or greater than approximately1 cm. One or more (or even each) of the fragments may have a width (ordiameter, or length, or other dimension) less than approximately 5 cm,less than approximately 4 cm, less than approximately 3 cm, less thanapproximately 2 cm, less than approximately 1.5 cm, or less thanapproximately 1 cm. The at least a portion of the AlN ceramic mayinclude, consist essentially of, or consist of one or more of thefragments.

A crystalline orientation of the seed crystal may be substantiallyparallel to a c-axis. The first crucible and the second crucible may bethe same crucible or different crucibles. The growth rate may be atleast 0.1 mm/hour, at least 0.2 mm/hour, at least 0.3 mm/hour, at least0.4 mm/hour, at least 0.5 mm/hour, at least 0.7 mm/hour, or at least 1mm/hour. The growth rate may be at most 3 mm/hour, at most 2.5 mm/hour,at most 2 mm/hour, at most 1.5 mm/hour, or at most 1 mm/hour. The soaktime may be at least approximately 2 hours, at least approximately 3hours, at least approximately 4 hours, at least approximately 5 hours,or at least approximately 6 hours. The soak time may be at mostapproximately 9 hours, at most approximately 8 hours, at mostapproximately 7 hours, at most approximately 6 hours, or at mostapproximately 5 hours. The soak time may be approximately 5 hours. Thediameter of the seed crystal may be approximately 25 mm or larger,approximately 30 mm or larger, approximately 35 mm or larger,approximately 40 mm or larger, approximately 45 mm or larger, orapproximately 50 mm or larger. The diameter of the seed crystal may beapproximately 150 mm or smaller, approximately 100 mm or smaller,approximately 75 mm or smaller, or approximately 50 mm or smaller.

The AlN boule may be sliced to form a single-crystalline AlN substratehaving a diameter of at least 25 mm, at least 30 mm, at least 35 mm, atleast 40 mm, at least 45 mm, or at least 50 mm. The diameter of thesingle-crystalline AlN substrate may be approximately 150 mm or smaller,approximately 100 mm or smaller, approximately 75 mm or smaller, orapproximately 50 mm or smaller. The diameter of the single-crystallineAlN substrate may be approximately 50 mm. A light-emitting device may befabricated over at least a portion of the AlN substrate. Thelight-emitting device may be configured to emit ultraviolet light. Afterforming at least a portion of the light-emitting device (e.g., all or aportion of an epitaxial light-emitting layer structure), at least aportion (or even all) of the AlN substrate may be removed from thelight-emitting device. The light-emitting device may include, consistessentially of, or consist of a light-emitting diode and/or a laser.

Oxygen and/or carbon may be gettered during formation of thesingle-crystalline AlN boule. The oxygen and/or carbon may be getteredwith a gettering material introduced into the second crucible and/or thefurnace prior to and/or during formation of the single-crystalline AlNboule. The gettering material may have a melting point greater than thegrowth temperature and/or a eutectic melting point with AlN greater thanthe growth temperature. The gettering material may include, consistessentially of, or consist of boron, iridium, niobium, molybdenum,tantalum, tungsten, and/or rhenium. The bulk polycrystalline AlN ceramicmay have less than approximately 1% excess Al and/or an oxygenconcentration less than 2×10¹⁹ cm⁻³. Embodiments of the invention mayinclude AlN boules and/or substrates formed in accordance with any ofthe above methods.

In another aspect, embodiments of the invention feature a single-crystalAlN substrate having (i) a diameter of at least approximately 50 mm and(ii) an ultraviolet (UV) transparency metric ranging from approximately5 cm³ to approximately 5000 cm³ at a wavelength of interest of 265 nm.The UV transparency metric is defined in cm³ as

$\frac{d}{10 \times FWHM \times a^{2}},$

where d is a diameter of the AlN substrate in mm, FWHM is a full-widthat half-maximum of an x-ray diffraction curve of the AlN substrate inradians, and a is an absorption coefficient of the AlN substrate at thewavelength of interest.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The diameter of the AlN substrate maybe approximately 50 mm. The diameter of the AlN substrate may be atleast approximately 60 mm, at least approximately 65 mm, at leastapproximately 70 mm, at least approximately 75 mm, at leastapproximately 80 mm, at least approximately 85 mm, at leastapproximately 90 mm, at least approximately 95 mm, or at leastapproximately 100 mm. The diameter of the AlN substrate may be nogreater than approximately 150 mm, no greater than approximately 125 mm,no greater than approximately 110 mm, or no greater than approximately100 mm. The thermal conductivity of the AlN substrate may be, at roomtemperature, approximately 250 W/m·K or greater, approximately 270 W/m·Kor greater, approximately 290 W/m·K or greater, approximately 300 W/m·Kor greater, or approximately 320 W/m·K or greater. The thermalconductivity of the AlN substrate may be, at room temperature,approximately 400 W/m·K or less, approximately 350 W/m·K or less, orapproximately 300 W/m·K or less. A crystalline orientation of the AlNsubstrate may be substantially parallel to a c-axis. A crystallineorientation of the AlN substrate may be angled at least approximately10° relative to a c-axis, at least approximately 12° relative to ac-axis, at least approximately 15° relative to a c-axis, or at leastapproximately 20° relative to a c-axis. A crystalline orientation of theAlN substrate may be angled at most approximately 30° relative to ac-axis, at most approximately 25° relative to a c-axis, at mostapproximately 20° relative to a c-axis, or at most approximately 15°relative to a c-axis.

A light-emitting device may be disposed over the AlN substrate. Thelight-emitting device may be configured to emit ultraviolet light. Thelight-emitting device may include, consist essentially of, or consist ofa light-emitting diode and/or a laser. A density of threading edgedislocations in the AlN substrate may be less than 5×10⁵ cm⁻², less than1×10⁵ cm⁻², less than 5×10⁴ cm⁻², less than 1×10⁴ cm⁻², less than 5×10³cm⁻², or less than 1×10³ cm⁻². A density of threading edge dislocationsin the AlN substrate may be more than 10 cm⁻², more than 100 cm⁻², morethan 500 cm⁻², or more than 1000 cm⁻². A density of threading screwdislocations in the AlN substrate may be less than 100 cm⁻², less than50 cm⁻², less than 10 cm⁻², less than 5 cm⁻², or less than 1 cm⁻². Adensity of threading screw dislocations in the AlN substrate may be morethan 0.1 cm⁻², more than 0.5 cm⁻², more than 1 cm⁻², more than 2 cm⁻²,or more than 5 cm⁻².

A silicon concentration of the AlN substrate may be less than 1×10¹⁹cm⁻³, less than 5×10¹⁸ cm⁻³, less than 1×10¹⁸ cm⁻³, less than 5×10¹⁷cm⁻³, less than 3×10¹⁷ cm⁻³, less than 1×10¹⁷ cm⁻³, less than 5×10¹⁶cm⁻³, or less than 1×10¹⁶ cm⁻³. A silicon concentration of the AlNsubstrate may be more than 1×10¹⁴ cm⁻³, more than 5×10¹⁴ cm⁻³, more than1×10¹⁵ cm⁻³, more than 5×10¹⁵ cm⁻³, more than 1×10¹⁶ cm⁻³, more than5×10¹⁶ cm⁻³, or more than 1×10¹⁷ cm⁻³. An oxygen concentration of theAlN substrate may be less than 1×10¹⁹ cm⁻³, less than 5×10¹⁸ cm⁻³, lessthan 1×10¹⁸ cm⁻³, less than 5×10¹⁷ cm⁻³, less than 3×10¹⁷ cm⁻³, lessthan 1×10¹⁷ cm⁻³, less than 5×10¹⁶ cm⁻³, or less than 1×10¹⁶ cm⁻³. Anoxygen concentration of the AlN substrate may be more than 1×10¹⁴ cm⁻³,more than 5×10¹⁴ cm⁻³, more than 1×10¹⁵ cm⁻³, more than 5×10¹⁵ cm⁻³,more than 1×10¹⁶ cm⁻³, more than 5×10¹⁶ cm⁻³, or more than 1×10¹⁷ cm⁻³.A carbon concentration of the AlN substrate may be less than 1×10¹⁹cm⁻³, less than 5×10¹⁸ cm⁻³, less than 1×10¹⁸ cm⁻³, less than 5×10¹⁷cm⁻³, less than 3×10¹⁷ cm⁻³, less than 1×10¹⁷ cm⁻³, less than 5×10¹⁶cm⁻³, or less than 1×10¹⁶ cm⁻³. A carbon concentration of the AlNsubstrate may be more than 1×10¹⁴ cm⁻³, more than 5×10¹⁴ cm⁻³, more than1×10¹⁵ cm⁻³, more than 5×10¹⁵ cm⁻³, more than 1×10¹⁶ cm⁻³, more than5×10¹⁶ cm⁻³, or more than 1×10¹⁷ cm⁻³. The ratio of the carbonconcentration of the AlN substrate to the oxygen concentration of theAlN substrate may be less than 1, less than 0.9, less than 0.8, lessthan 0.7, less than 0.6, less than 0.5, less than 0.4, or less than 0.3.The ratio of the carbon concentration of the AlN substrate to the oxygenconcentration of the AlN substrate may be more than 0.1, more than 0.2,more than 0.3, or more than 0.4.

In yet another aspect, embodiments of the invention feature asingle-crystal AlN substrate having an ultraviolet (UV) transparencymetric ranging from approximately 20 cm³ to approximately 5000 cm³ at awavelength of interest of 265 nm. The UV transparency metric is definedin cm³ as

$\frac{d}{10 \times FWHM \times a^{2}},$

where d is a diameter of the AlN substrate in mm, FWHM is a full-widthat half-maximum of an x-ray diffraction curve of the AlN substrate inradians, and a is an absorption coefficient of the AlN substrate at thewavelength of interest.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The diameter of the AlN substrate maybe approximately 50 mm. The diameter of the AlN substrate may be atleast approximately 60 mm, at least approximately 65 mm, at leastapproximately 70 mm, at least approximately 75 mm, at leastapproximately 80 mm, at least approximately 85 mm, at leastapproximately 90 mm, at least approximately 95 mm, or at leastapproximately 100 mm. The diameter of the AlN substrate may be nogreater than approximately 150 mm, no greater than approximately 125 mm,no greater than approximately 110 mm, or no greater than approximately100 mm. The thermal conductivity of the AlN substrate may be, at roomtemperature, approximately 250 W/m·K or greater, approximately 270 W/m·Kor greater, approximately 290 W/m·K or greater, approximately 300 W/m·Kor greater, or approximately 320 W/m·K or greater. The thermalconductivity of the AlN substrate may be, at room temperature,approximately 400 W/m·K or less, approximately 350 W/m·K or less, orapproximately 300 W/m·K or less. A crystalline orientation of the AlNsubstrate may be substantially parallel to a c-axis. A crystallineorientation of the AlN substrate may be angled at least approximately10° relative to a c-axis, at least approximately 12° relative to ac-axis, at least approximately 15° relative to a c-axis, or at leastapproximately 20° relative to a c-axis. A crystalline orientation of theAlN substrate may be angled at most approximately 30° relative to ac-axis, at most approximately 25° relative to a c-axis, at mostapproximately 20° relative to a c-axis, or at most approximately 15°relative to a c-axis.

A light-emitting device may be disposed over the AlN substrate. Thelight-emitting device may be configured to emit ultraviolet light. Thelight-emitting device may include, consist essentially of, or consist ofa light-emitting diode and/or a laser. A density of threading edgedislocations in the AlN substrate may be less than 5×10⁵ cm⁻², less than1×10⁵ cm⁻², less than 5×10⁴ cm⁻², less than 1×10⁴ cm⁻², less than 5×10³cm⁻², or less than 1×10³ cm⁻². A density of threading edge dislocationsin the AlN substrate may be more than 10 cm⁻², more than 100 cm⁻², morethan 500 cm⁻², or more than 1000 cm⁻². A density of threading screwdislocations in the AlN substrate may be less than 100 cm⁻², less than50 cm⁻², less than 10 cm⁻², less than 5 cm⁻², or less than 1 cm⁻². Adensity of threading screw dislocations in the AlN substrate may be morethan 0.1 cm⁻², more than 0.5 cm⁻², more than 1 cm⁻², more than 2 cm⁻²,or more than 5 cm⁻².

A silicon concentration of the AlN substrate may be less than 1×10¹⁹cm⁻³, less than 5×10¹⁸ cm⁻³, less than 1×10¹⁸ cm⁻³, less than 5×10¹⁷cm⁻³, less than 3×10¹⁷ cm⁻³, less than 1×10¹⁷ cm⁻³, less than 5×10¹⁶cm⁻³, or less than 1×10¹⁶ cm⁻³. A silicon concentration of the AlNsubstrate may be more than 1×10¹⁴ cm⁻³, more than 5×10¹⁴ cm⁻³, more than1×10¹⁵ cm⁻³, more than 5×10¹⁵ cm⁻³, more than 1×10¹⁶ cm⁻³, more than5×10¹⁶ cm⁻³, or more than 1×10¹⁷ cm⁻³. An oxygen concentration of theAlN substrate may be less than 1×10¹⁹ cm⁻³, less than 5×10¹⁸ cm⁻³, lessthan 1×10¹⁸ cm⁻³, less than 5×10¹⁷ cm⁻³, less than 3×10¹⁷ cm⁻³, lessthan 1×10¹⁷ cm⁻³, less than 5×10¹⁶ cm⁻³, or less than 1×10¹⁶ cm⁻³. Anoxygen concentration of the AlN substrate may be more than 1×10¹⁴ cm⁻³,more than 5×10¹⁴ cm⁻³, more than 1×10¹⁵ cm⁻³, more than 5×10¹⁵ cm⁻³,more than 1×10¹⁶ cm⁻³, more than 5×10¹⁶ cm⁻³, or more than 1×10¹⁷ cm⁻³.A carbon concentration of the AlN substrate may be less than 1×10¹⁹cm⁻³, less than 5×10¹⁸ cm⁻³, less than 1×10¹⁸ cm⁻³, less than 5×10¹⁷cm⁻³, less than 3×10¹⁷ cm⁻³, less than 1×10¹⁷ cm⁻³, less than 5×10¹⁶cm⁻³, or less than 1×10¹⁶ cm⁻³. A carbon concentration of the AlNsubstrate may be more than 1×10¹⁴ cm⁻³, more than 5×10¹⁴ cm⁻³, more than1×10¹⁵ cm⁻³, more than 5×10¹⁵ cm⁻³, more than 1×10¹⁶ cm⁻³, more than5×10¹⁶ cm⁻³, or more than 1×10¹⁷ cm⁻³. The ratio of the carbonconcentration of the AlN substrate to the oxygen concentration of theAlN substrate may be less than 1, less than 0.9, less than 0.8, lessthan 0.7, less than 0.6, less than 0.5, less than 0.4, or less than 0.3.The ratio of the carbon concentration of the AlN substrate to the oxygenconcentration of the AlN substrate may be more than 0.1, more than 0.2,more than 0.3, or more than 0.4.

In another aspect, embodiments of the invention feature asingle-crystalline AlN boule having a diameter of approximately 50 mm orlarger and a length of approximately 15 mm or larger. The ultraviolet(UV) transparency of the AlN boule is less than 60 cm⁻¹ for a wavelengthrange of approximately 220 nm to approximately 480 nm. The oxygenconcentration of the AlN boule is less than 4×10¹⁷ cm⁻³, and/or thecarbon concentration of the AlN boule is less than 4×10¹⁷ cm⁻³. Theratio of the carbon concentration of the AlN boule to the oxygenconcentration of the AlN boule is less than 1.0.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The UV transparency, for thewavelength range of approximately 220 nm to approximately 480 nm, may beless than 55 cm⁻¹, less than 50 cm⁻¹, less than 45 cm⁻¹, less than 40cm⁻¹, less than 35 cm⁻¹, or less than 30 cm⁻¹. The UV transparency, forthe wavelength range of approximately 220 nm to approximately 480 nm,may be more than 10 cm⁻¹, more than 15 cm⁻¹, more than 20 cm⁻¹, or morethan 25 cm⁻¹. A silicon concentration of the AlN boule may be less than1×10¹⁹ cm⁻³, less than 5×10¹⁸ cm⁻³, less than 1×10¹⁸ cm⁻³, less than5×10¹⁷ cm⁻³, less than 3×10¹⁷ cm⁻³, less than 1×10¹⁷ cm⁻³, less than5×10¹⁶ cm⁻³, or less than 1×10¹⁶ cm⁻³. A silicon concentration of theAlN boule may be more than 1×10¹⁴ cm⁻³, more than 5×10¹⁴ cm⁻³, more than1×10¹⁵ cm⁻³, more than 5×10¹⁵ cm⁻³, more than 1×10¹⁶ cm⁻³, more than5×10¹⁶ cm⁻³, or more than 1×10¹⁷ cm⁻³. An oxygen concentration of theAlN boule may be less than 3×10¹⁷ cm⁻³, less than 1×10¹⁷ cm⁻³, less than5×10¹⁶ cm⁻³, or less than 1×10¹⁶ cm⁻³. An oxygen concentration of theAlN boule may be more than 1×10¹⁴ cm⁻³, more than 5×10¹⁴ cm⁻³, more than1×10¹⁵ cm⁻³, more than 5×10¹⁵ cm⁻³, more than 1×10¹⁶ cm⁻³, more than5×10¹⁶ cm⁻³, or more than 1×10¹⁷ cm⁻³. A carbon concentration of the AlNboule may be less than 3×10¹⁷ cm⁻³, less than 1×10¹⁷ cm⁻³, less than5×10¹⁶ cm⁻³, more or less than 1×10¹⁶ cm⁻³. A carbon concentration ofthe AlN boule may be more than 1×10¹⁴ cm⁻³, more than 5×10¹⁴ cm⁻³, morethan 1×10¹⁵ cm⁻³, more than 5×10¹⁵ cm⁻³, more than 1×10¹⁶ cm⁻³, morethan 5×10¹⁶ cm⁻³, or more than 1×10¹⁷ cm⁻³. The ratio of the carbonconcentration of the AlN boule to the oxygen concentration of the AlNboule may be less than 0.9, less than 0.8, less than 0.7, less than 0.6,less than 0.5, less than 0.4, or less than 0.3. The ratio of the carbonconcentration of the AlN boule to the oxygen concentration of the AlNboule may be more than 0.1, more than 0.2, more than 0.3, or more than0.4.

The length of the boule may be approximately 17 mm or larger,approximately 20 mm or larger, approximately 25 mm or larger,approximately 30 mm or larger, or approximately 35 mm or larger. Thelength of the boule may be approximately 50 mm or less, approximately 45mm or less, approximately 40 mm or less, or approximately 35 mm or less.The diameter of the AlN boule may be approximately 50 mm. The diameterof the AlN boule may be at least approximately 60 mm, at leastapproximately 65 mm, at least approximately 70 mm, at leastapproximately 75 mm, at least approximately 80 mm, at leastapproximately 85 mm, at least approximately 90 mm, at leastapproximately 95 mm, or at least approximately 100 mm. The diameter ofthe AlN boule may be no greater than approximately 150 mm, no greaterthan approximately 125 mm, no greater than approximately 110 mm, or nogreater than approximately 100 mm. The thermal conductivity of the AlNboule may be, at room temperature, approximately 250 W/m·K or greater,approximately 270 W/m·K or greater, approximately 290 W/m·K or greater,approximately 300 W/m·K or greater, or approximately 320 W/m·K orgreater. The thermal conductivity of the AlN boule may be, at roomtemperature, approximately 400 W/m·K or less, approximately 350 W/m·K orless, or approximately 300 W/m·K or less.

A full-width at half-maximum (FWHM) of an x-ray diffraction curve of theAlN boule may be less than 85 arcsec, less than 80 arcsec, less than 75arcsec, less than 70 arcsec, less than 65 arcsec, less than 60 arcsec,less than 55 arcsec, less than 50 arcsec, less than 45 arcsec, or lessthan 40 arcsec. A FWHM of an x-ray diffraction curve of the AlN boulemay be more than 10 arcsec, more than 15 arcsec, more than 20 arcsec,more than 25 arcsec, more than 30 arcsec, more than 35 arcsec, more than40 arc sec, or more than 45 arcsec. A density of threading edgedislocations in the AlN boule may be less than 5×10⁵ cm⁻², less than1×10⁵ cm⁻², less than 5×10⁴ cm⁻², less than 1×10⁴ cm⁻², less than 5×10³cm⁻², or less than 1×10³ cm⁻². A density of threading edge dislocationsin the AlN boule may be more than 10 cm⁻², more than 100 cm⁻², more than500 cm⁻², or more than 1000 cm⁻². A density of threading screwdislocations in the AlN boule may be less than 100 cm⁻², less than 50cm⁻², less than 10 cm⁻², less than 5 cm⁻², or less than 1 cm⁻². Adensity of threading screw dislocations in the AlN boule may be morethan 0.1 cm⁻², more than 0.5 cm⁻², more than 1 cm⁻², more than 2 cm⁻²,or more than 5 cm⁻². The UV transparency of the boule, for thewavelength range of approximately 350 nm to approximately 480 nm, may beless than 25 cm⁻¹, less than 20 cm⁻¹, less than 15 cm⁻¹, less than 10cm⁻¹, less than 8 cm⁻¹, or less than 5 cm⁻¹. The UV transparency, forthe wavelength range of approximately 350 nm to approximately 480 nm,may be more than 1 cm⁻¹, more than 3 cm⁻¹, more than 5 cm⁻¹, or morethan 8 cm⁻¹.

In yet another aspect, embodiments of the invention feature a method ofimproving ultraviolet (UV) transparency of a single-crystal AlN bulkcrystal. The AlN bulk crystal is heated to an annealing temperature ofat least 1800° C., at least 1900° C., at least 1950° C., or at least2000° C. Thereafter, the AlN bulk crystal is cooled via a cooling cycle.The cooling cycle includes, consists essentially of, or consists of (i)cooling the AlN bulk crystal from the annealing temperature to a firsttemperature ranging from 1450° C. to 2150° C. over a first time rangingfrom 10 minutes to 90 minutes, and (ii) thereafter, cooling the AlN bulkcrystal from the first temperature to a second temperature ranging from1000° C. to 1650° C. over a second time ranging from 10 seconds to 10minutes, or (i) cooling the AlN bulk crystal from the annealingtemperature to a third temperature ranging from 1450° C. to 2150° C.over a third time ranging from 10 seconds to 10 minutes, and (ii)thereafter, cooling the AlN bulk crystal from the third temperature to afourth temperature ranging from 1000° C. to 1650° C. over a fourth timeranging from 10 minutes to 90 minutes. Thereafter, the AlN bulk crystalis cooled to approximately room temperature.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. After heating the AlN bulk crystal tothe annealing temperature and before cooling the AlN bulk crystal viathe cooling cycle, the AlN bulk crystal may be maintained at theannealing temperature for a soak time. The soak time may be at leastapproximately 1 hour, at least approximately 2 hours, at leastapproximately 3 hours, at least approximately 4 hours, at leastapproximately 5 hours, or at least approximately 6 hours. The soak timemay be at most approximately 10 hours, at most approximately 9 hours, atmost approximately 8 hours, at most approximately 7 hours, at mostapproximately 6 hours, or at most approximately 5 hours. The soak timemay be approximately 5 hours. The diameter of at least a portion (oreven all) of the AlN bulk crystal may be at least approximately 35 mm,at least approximately 50 mm, at least approximately 75 mm, or at leastapproximately 100 mm. The diameter of at least a portion (or even all)of the AlN bulk crystal may be at most approximately 150 mm, at mostapproximately 125 mm, at most approximately 100 mm, or at mostapproximately 75 mm. The diameter of at least a portion (or even all) ofthe AlN bulk crystal may be approximately 50 mm.

A light-emitting device may be fabricated over at least a portion of theAlN bulk crystal. The light-emitting device may be configured to emitultraviolet light. After forming at least a portion of thelight-emitting device (e.g., all or a portion of an epitaxiallight-emitting layer structure), at least a portion (or even all) of theAlN bulk crystal may be removed from the light-emitting device. Thelight-emitting device may include, consist essentially of, or consist ofa light-emitting diode and/or a laser. Embodiments of the invention mayinclude AlN bulk crystals formed in accordance with any of the methodsdescribed above.

In another aspect, embodiments of the invention feature a method ofimproving ultraviolet (UV) transparency of a single-crystal AlN bulkcrystal. The AlN bulk crystal is heated to an annealing temperature ofat least 1800° C., at least 1900° C., at least 1950° C., or at least2000° C. Thereafter, the AlN bulk crystal is cooled via a cooling cycle.The cooling cycle includes, consists essentially of, or consists ofcooling the AlN bulk crystal from the annealing temperature to a firsttemperature ranging from 1000° C. to 1650° C. over a time period rangingfrom 1 hour to 10 hours, and thereafter, cooling the AlN bulk crystal toapproximately room temperature.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. After heating the AlN bulk crystal tothe annealing temperature and before cooling the AlN bulk crystal viathe cooling cycle, the AlN bulk crystal may be maintained at theannealing temperature for a soak time. The soak time may be at leastapproximately 1 hour, at least approximately 2 hours, at leastapproximately 3 hours, at least approximately 4 hours, at leastapproximately 5 hours, or at least approximately 6 hours. The soak timemay be at most approximately 10 hours, at most approximately 9 hours, atmost approximately 8 hours, at most approximately 7 hours, at mostapproximately 6 hours, or at most approximately 5 hours. The soak timemay be approximately 5 hours. The diameter of at least a portion (oreven all) of the AlN bulk crystal may be at least approximately 35 mm,at least approximately 50 mm, at least approximately 75 mm, or at leastapproximately 100 mm. The diameter of at least a portion (or even all)of the AlN bulk crystal may be at most approximately 150 mm, at mostapproximately 125 mm, at most approximately 100 mm, or at mostapproximately 75 mm. The diameter of at least a portion (or even all) ofthe AlN bulk crystal may be approximately 50 mm.

A light-emitting device may be fabricated over at least a portion of theAlN bulk crystal. The light-emitting device may be configured to emitultraviolet light. After forming at least a portion of thelight-emitting device (e.g., all or a portion of an epitaxiallight-emitting layer structure), at least a portion (or even all) of theAlN bulk crystal may be removed from the light-emitting device. Thelight-emitting device may include, consist essentially of, or consist ofa light-emitting diode and/or a laser. Embodiments of the invention mayinclude AlN bulk crystals formed in accordance with any of the methodsdescribed above.

In yet another aspect, embodiments of the invention feature a method offorming a polycrystalline AlN source. A bulk polycrystalline AlN ceramicis provided. At least a portion of the AlN ceramic is disposed into acrucible. The at least a portion of the AlN ceramic is annealed anddensified in the crucible, thereby forming a polycrystalline AlN source.The annealing and densifying includes, consists essentially of, orconsists of (i) heating the at least a portion of the AlN ceramic at afirst temperature ranging from 1100° C. to 1900° C. for a first timeranging from 2 hours to 25 hours, and (ii) thereafter, heating the atleast a portion of the AlN ceramic at a second temperature ranging from1900° C. to 2250° C. for a second time ranging from 3 hours to 15 hours,or (i) heating the at least a portion of the AlN ceramic during atemperature ramp to a third temperature ranging from 1900° C. to 2250°C. over a third time ranging from 5 hours to 25 hours, and (ii)thereafter, heating the at least a portion of the AlN ceramic at afourth temperature ranging from 1900° C. to 2250° C. for a fourth timeranging from 3 hours to 25 hours. The AlN source is cooled toapproximately room temperature.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. Prior to disposing the at least aportion of the AlN ceramic into the crucible, the AlN ceramic may befragmented into fragments. One or more (or even each) of the fragmentsmay have a width (or diameter, or length, or other dimension) greaterthan approximately 0.1 cm, greater than approximately 0.2 cm, greaterthan approximately 0.3 cm, greater than approximately 0.4 cm, greaterthan approximately 0.5 cm, greater than approximately 0.7 cm, or greaterthan approximately 1 cm. One or more (or even each) of the fragments mayhave a width (or diameter, or length, or other dimension) less thanapproximately 5 cm, less than approximately 4 cm, less thanapproximately 3 cm, less than approximately 2 cm, less thanapproximately 1.5 cm, or less than approximately 1 cm. The at least aportion of the AlN ceramic may include, consist essentially of, orconsist of one or more of the fragments. The bulk polycrystalline AlNceramic may have less than approximately 1% excess Al and/or an oxygenconcentration less than 2×10¹⁹ cm⁻³. Embodiments of the invention mayinclude polycrystalline AlN sources formed in accordance with any of theabove methods.

In another aspect, embodiments of the invention feature a method offorming single-crystal aluminum nitride (AlN). A bulk polycrystallineAlN ceramic is provided. At least a portion of the AlN ceramic isdisposed into a first crucible. The at least a portion of the AlNceramic is annealed and densified in the first crucible, thereby forminga polycrystalline AlN source. The annealing and densifying includes,consists essentially of, or consists of (i) heating the at least aportion of the AlN ceramic at a first temperature ranging from 1100° C.to 1900° C. for a first time ranging from 2 hours to 25 hours, and (ii)thereafter, heating the at least a portion of the AlN ceramic at asecond temperature ranging from 1900° C. to 2250° C. for a second timeranging from 3 hours to 15 hours, or (i) heating the at least a portionof the AlN ceramic during a temperature ramp to a third temperatureranging from 1900° C. to 2250° C. over a third time ranging from 5 hoursto 25 hours, and (ii) thereafter, heating the at least a portion of theAlN ceramic at a fourth temperature ranging from 1900° C. to 2250° C.for a fourth time ranging from 3 hours to 25 hours. The AlN source iscooled to approximately room temperature. A second crucible is disposedwithin a furnace. The second crucible contains the AlN source and a seedcrystal that includes, consists essentially of, or consists ofsingle-crystal AlN. The second crucible is heated with the furnace to agrowth temperature of at least 2000° C. The second crucible ismaintained at the growth temperature for a soak time ranging from 1 hourto 10 hours. After the soak time, while the second crucible is at thegrowth temperature, (i) vapor including, consisting essentially of, orconsisting of aluminum and nitrogen is condensed on the seed crystal,thereby forming a single-crystalline AlN boule extending from the seedcrystal, and (ii) the second crucible is moved relative to the furnace.The growth rate of at least a portion of the AlN boule is approximatelyequal to a rate of relative motion between the second crucible and thefurnace. Thereafter, the AlN boule is cooled via a cooling cycle. Thecooling cycle includes, consists essentially of, or consists of coolingthe AlN boule from the growth temperature to a fifth temperature rangingfrom 1000° C. to 1650° C. over a fifth time ranging from 1 hour to 10hours, and thereafter, cooling the AlN boule to approximately roomtemperature.

Embodiments of the invention may include one or more of the following inany of a variety of combinations. The growth temperature may beapproximately 2300° C. or less, approximately 2200° C. or less, orapproximately 2100° C. or less. The diameter of at least a portion (oreven all) of the AlN boule may be at least approximately 35 mm, at leastapproximately 50 mm, at least approximately 75 mm, or at leastapproximately 100 mm. The diameter of at least a portion (or even all)of the AlN boule may be at most approximately 150 mm, at mostapproximately 125 mm, at most approximately 100 mm, or at mostapproximately 75 mm. The diameter of at least a portion (or even all) ofthe AlN boule may be approximately 50 mm. Oxygen (e.g., oxygen gas or anoxygen-containing gas) may be introduced into the second crucible duringformation of the single-crystalline AlN boule extending from the seedcrystal. Prior to disposing the at least a portion of the AlN ceramicinto the first crucible, the AlN ceramic may be fragmented intofragments. One or more (or even each) of the fragments may have a width(or diameter, or length, or other dimension) greater than approximately0.1 cm, greater than approximately 0.2 cm, greater than approximately0.3 cm, greater than approximately 0.4 cm, greater than approximately0.5 cm, greater than approximately 0.7 cm, or greater than approximately1 cm. One or more (or even each) of the fragments may have a width (ordiameter, or length, or other dimension) less than approximately 5 cm,less than approximately 4 cm, less than approximately 3 cm, less thanapproximately 2 cm, less than approximately 1.5 cm, or less thanapproximately 1 cm. The at least a portion of the AlN ceramic mayinclude, consist essentially of, or consist of one or more of thefragments.

A crystalline orientation of the seed crystal may be substantiallyparallel to a c-axis. The first crucible and the second crucible may bethe same crucible or different crucibles. The growth rate may be atleast 0.1 mm/hour, at least 0.2 mm/hour, at least 0.3 mm/hour, at least0.4 mm/hour, at least 0.5 mm/hour, at least 0.7 mm/hour, or at least 1mm/hour. The growth rate may be at most 3 mm/hour, at most 2.5 mm/hour,at most 2 mm/hour, at most 1.5 mm/hour, or at most 1 mm/hour. The soaktime may be at least approximately 2 hours, at least approximately 3hours, at least approximately 4 hours, at least approximately 5 hours,or at least approximately 6 hours. The soak time may be at mostapproximately 9 hours, at most approximately 8 hours, at mostapproximately 7 hours, at most approximately 6 hours, or at mostapproximately 5 hours. The soak time may be approximately 5 hours. Thediameter of the seed crystal may be approximately 25 mm or larger,approximately 30 mm or larger, approximately 35 mm or larger,approximately 40 mm or larger, approximately 45 mm or larger, orapproximately 50 mm or larger. The diameter of the seed crystal may beapproximately 150 mm or smaller, approximately 100 mm or smaller,approximately 75 mm or smaller, or approximately 50 mm or smaller.

The AlN boule may be sliced to form a single-crystalline AlN substratehaving a diameter of at least 25 mm, at least 30 mm, at least 35 mm, atleast 40 mm, at least 45 mm, or at least 50 mm. The diameter of thesingle-crystalline AlN substrate may be approximately 150 mm or smaller,approximately 100 mm or smaller, approximately 75 mm or smaller, orapproximately 50 mm or smaller. The diameter of the single-crystallineAlN substrate may be approximately 50 mm. A light-emitting device may befabricated over at least a portion of the AlN substrate. Thelight-emitting device may be configured to emit ultraviolet light. Afterforming at least a portion of the light-emitting device (e.g., all or aportion of an epitaxial light-emitting layer structure), at least aportion (or even all) of the AlN substrate may be removed from thelight-emitting device. The light-emitting device may include, consistessentially of, or consist of a light-emitting diode and/or a laser.

Oxygen and/or carbon may be gettered during formation of thesingle-crystalline AlN boule. The oxygen and/or carbon may be getteredwith a gettering material introduced into the second crucible and/or thefurnace prior to and/or during formation of the single-crystalline AlNboule. The gettering material may have a melting point greater than thegrowth temperature and/or a eutectic melting point with AlN greater thanthe growth temperature. The gettering material may include, consistessentially of, or consist of boron, iridium, niobium, molybdenum,tantalum, tungsten, and/or rhenium. The bulk polycrystalline AlN ceramicmay have less than approximately 1% excess Al and/or an oxygenconcentration less than 2×10¹⁹ cm⁻³. Embodiments of the invention mayinclude AlN boules and/or substrates formed in accordance with any ofthe above methods.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations. As used herein, theterms “approximately,” “about,” and “substantially” mean±10%, and insome embodiments, ±5%. All numerical ranges specified herein areinclusive of their endpoints unless otherwise specified. The term“consists essentially of” means excluding other materials thatcontribute to function, unless otherwise defined herein. Nonetheless,such other materials may be present, collectively or individually, intrace amounts.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention. In the followingdescription, various embodiments of the present invention are describedwith reference to the following drawings, in which:

FIG. 1A is a graph of UV absorption coefficients, as a function ofwavelength, for AlN wafers produced utilizing conventional growthtechniques;

FIGS. 1B-1D are schematic diagrams of a reactor utilized for theformation of polycrystalline source material in accordance with variousembodiments of the invention;

FIGS. 1E and 1F are schematic diagrams of a crucible utilized for theformation of polycrystalline source material in accordance with variousembodiments of the invention;

FIGS. 1G and 1H are graphs of example heat treatments for the formationof polycrystalline source material in accordance with variousembodiments of the invention;

FIG. 2 is a schematic diagram of an apparatus for the growth ofsingle-crystal AlN in accordance with various embodiments of theinvention;

FIG. 3 is a graph of example cooling cycles for single-crystal AlN inaccordance with various embodiments of the invention;

FIG. 4A is graph of UV absorption coefficient, as a function ofwavelength, for single-crystal AlN grown in accordance with variousembodiments of the invention;

FIG. 4B is a micrograph of an etch pit density measurement ofsingle-crystal AlN grown in accordance with various embodiments of theinvention;

FIG. 4C is an x-ray topography map of single-crystal AlN grown inaccordance with various embodiments of the invention;

FIGS. 4D and 4E are x-ray rocking curves of single-crystal AlN grown inaccordance with various embodiments of the invention;

FIG. 4F is a graph of thermal conductivity of single-crystal AlN grownin accordance with various embodiments of the invention;

FIG. 5A is a series of micrographs of various single-crystal AlN wafers,grown in accordance with various embodiments of the invention; for eachwafer, a view under normal illumination and a view of PL luminescenceare shown;

FIG. 5B is a table of data showing Si, 0, and C concentrations, as wellas UV absorption coefficients, for the measurement sites indicated bythe circles on FIG. 5A;

FIGS. 6A and 6B are graphs showing the relationship between Cconcentration and C-to-O ratio with UV absorption coefficient ofsingle-crystal AlN wafers grown in accordance with various embodimentsof the invention;

FIG. 7 is a graph of UV LED output power as a function of UVtransparency metric of the LED substrate in accordance with variousembodiments of the invention;

FIGS. 8A and 8B are schematic cross-sections of light-emitting devicesin accordance with various embodiments of the invention; and

FIGS. 9A and 9B are schematic cross-sections of light-emitting devicesafter substrate removal in accordance with various embodiments of theinvention.

DETAILED DESCRIPTION

Embodiments of the present invention enable the fabrication ofhigh-quality, highly UV-transparent single-crystal AlN bulk crystals(i.e., boules and/or substrates). In various embodiments, production ofsuch AlN bulk crystals begins with a two-stage process of fabricatinghighly stoichiometric polycrystalline AlN source material that may beutilized in a vapor-transport growth process (e.g.,sublimation-recondensation) to form the AlN bulk crystals. The formationof the AlN source material, in various embodiments, minimizesconcentrations of UV-transparency-compromising contaminants such ascarbon (C) and oxygen (O).

In various embodiments, the first stage of production of the AlN sourcematerial proceeds with the formation of a polycrystalline AlN ceramicfrom high-purity Al pellets as detailed in the '519 patent. For example,referring to FIGS. 1B-1F, a reactor 100 may be utilized in the formationof a polycrystalline AlN ceramic 195 that includes, consists essentiallyof, or consists of high-purity AlN. Reactor 100 may include a reactionvessel 110, which may be fabricated of double-walled stainless steel andmay be water cooled. Reaction vessel 110 may be capable of a maximuminternal gas pressure of approximately 45 pounds per square inch (psi),and may be evacuated, e.g., by a turbo pump 111 (backed by a mechanicalpump 112) to approximately 10⁻⁷ Torr. A feeder mechanism 120 isconnected to the top of reaction vessel 110, and may be evacuated andpressurized with the same gases and pressures as reaction vessel 110.Feeder mechanism 120 may be isolated from reaction vessel 110 by anisolation valve 122. Pellets (which may include, consist essentially of,or consist of high (e.g., five nines) purity undoped Al and may beshaped approximately cylindrically) released from feeder mechanism 120are directed to a crucible 130 by an upper funnel 132 and a lower funnel134.

In various embodiments, crucible 130 includes, consists essentially of,or consists of a bottom plug 136 and a foil wrap 137. Bottom plug 136may be approximately cylindrical with, e.g., a diameter of approximately2 inches and a height of approximately 0.5 inches. Bottom plug 136 mayinclude, consist essentially of, or consist of tungsten (W) or anotherhigh-melting-point material inert to AlN. Foil wrap 137 wraps aroundbottom plug 136, forming a cylinder open at the top and sealed at thebottom by bottom plug 136. Foil wrap 137 may include, consistessentially of, or consist of W, or another high melting point materialinert to AlN, and may have a thickness of approximately 0.001 inch. Inan embodiment, foil wrap 137 may be wrapped around bottom plug 136multiple times, e.g., a three-ply foil wrap 137 is formed by wrapping Wfoil around bottom plug 137 three times. Foil wrap 137 may be held inplace by wire 138. Wire 138 may include, consist essentially of, orconsist of a tungsten-rhenium alloy (e.g., 25% rhenium) and have athickness of approximately 0.01 inch.

As shown, crucible 130 is disposed within a reaction zone 140 and on topof a crucible stand 142. Both reaction zone 140 and crucible stand 142may include, consist essentially of, or consist of W. Lower funnel 134is disposed above the top opening of crucible 130, and may include,consist essentially of, or consist of W. Lower funnel 134 is shaped todirect pellets from feeder mechanism 120 and upper funnel 132 intocrucible 130.

Reactor 100 includes an inductive heating coil 150, which wraps aroundinsulation 160. Insulation 160 may include, consist essentially of, orconsist of bubble alumina available from Zircar Ceramics, Inc. ofFlorida, New York held within a quartz holder. Inductive heating coil150 may be a 10 kHz, 20 kilowatt inductive heating system available fromMesta Electronics, Inc. of N. Huntingdon, Pa., and may heat totemperatures up to approximately 2300° C. An optical pyrometer port 162enables the measurement of temperature inside the reaction zone definedby inductive heating coil 150 by pyrometry. Gas from a series of gastanks representatively indicated at 168 flows into reactor 100 from abottom inlet 170 and/or a top inlet 172. The gas may include, consistessentially of, or consist of nitrogen or forming gas, and is filteredby a gas filter 174 that reduces levels of contaminants such as oxygen,water vapor, and hydrocarbons to less than 10 ppb. A vertical drive 180is used to move crucible 130 in and out of the hot zone created byinductive heating coil 150. A conventional control station 190 includeselectronic controls and power supplies for all of the componentsassociated with reactor 100.

In order to form the polycrystalline ceramic 195, pellets are cleaned inpreparation for loading into feeder mechanism 120. First, the pelletsare sifted (with or without water) in order to remove oddly shapedpellets or small shavings. The pellets are then ultrasonically cleanedin methanol (e.g., for a time period of approximately 20 minutes),etched in hydrochloric acid (HCl) (e.g., for a time period ofapproximately 7 minutes), and rinsed several times (e.g. three times) indistilled water. After another ultrasonic clean in methanol (e.g., for atime period of approximately 20 minutes), the pellets are immersed in amixture of HF and HNO₃ (e.g., for a time period of approximately 2minutes) at room temperature. Finally, the pellets are rinsed indistilled water and multiple times in methanol, whereupon they may bestored in an inert or nitrogen atmosphere prior to loading in feedermechanism 120.

Crucible 130 is loaded into reactor 100, and pellets are loaded intofeeder mechanism 120. Reaction chamber 110 and feeder mechanism 120 areevacuated, e.g., to a pressure less than approximately 5×10⁻⁵ Torr, andrefilled with forming gas to a pressure of approximately 6 psi. Eithernitrogen (N₂) gas or forming gas flows into reaction chamber 110 frombottom inlet 170 and/or top inlet 172 at a rate of approximately 0.25lpm. The flow of gas provides a sufficient amount of nitrogen inreaction chamber 110 to convert the pellet(s) to AlN (as describedbelow). Inductive heating coil 150 may heat crucible 130 toapproximately 1900-2200° C., although even higher temperatures may beutilized. In an embodiment, inductive heating coil 150 heats crucible130 to approximately 2000-2050° C. Temperatures in this range have beenfound to be sufficient to totally react the pellets into stoichiometricAlN (which includes less than approximately 1% unreacted Al) and todrive off higher vapor pressure impurities that may be trapped withinpolycrystalline ceramic 130 and create optical absorptions. Thetemperature at crucible 130 may be measured by pyrometry through opticalpyrometer port 162. Once crucible 130 reaches the desired temperature,the temperature and gas flow conditions within reactor 100 are heldconstant for a pre-soak cycle (e.g., approximately 3 hours). Thepre-soak cleans crucible 130 and other parts of reactor 100 ofcontaminants, e.g., oxides, before the introduction of the Al pellets.

A reaction cycle is then performed to form polycrystalline ceramic 195.Pellets are dropped from feeder mechanism 120, through upper funnel 132and lower funnel 134, into crucible 130. The pellets may each weighapproximately 0.23 gram, and may be dropped at a rate of approximately 1every 90 seconds. Feeder mechanism 120 may incorporate an opticalcounter that counts actual pellet drops and may cycle feeder mechanism120 to drop an additional pellet in case of a loading error. The pelletsland on bottom plug 136 (or the portion of polycrystalline ceramic 195already produced thereon), melt, and react with the nitrogen gas to formpolycrystalline ceramic 195. Each subsequent pellet dropped from feedermechanism 120 reacts and increases the size and volume ofpolycrystalline ceramic 195. In an embodiment, substantially all of eachpellet reacts to form polycrystalline ceramic 195. After a desirednumber of pellets are reacted to form polycrystalline ceramic 195, thereaction gas flow rate and temperature are maintained for a time period(e.g., approximately 1 hour) to ensure that the reaction is complete.

After the reaction cycle, crucible 130 (and polycrystalline ceramic 195)may be cooled down to approximately room temperature at a positivenitrogen pressure (e.g., over a time period of approximately 1 hour).Thus formed, polycrystalline ceramic 195 includes, consists essentiallyof, or consists of high-purity AlN. In an embodiment, an oxygenconcentration (and/or concentration of other impurities such as boron ortransition metals) of polycrystalline ceramic 195 is less thanapproximately 400 ppm by weight, and may even be less than approximately100 ppm. In various embodiments, the oxygen concentration ofpolycrystalline ceramic 195, as measured by instrumental gas analysis(IGA), ranges from approximately 5.3×10¹⁹ cm⁻³ to approximately 6.1×10¹⁹cm⁻³. In various embodiments, the carbon concentration ofpolycrystalline ceramic 195, as measured by IGA, ranges fromapproximately 1.9×10²⁰ cm⁻³ to approximately 2.6×10²⁰ cm⁻³.

Polycrystalline ceramic 195 includes, consists essentially of, orconsists of AlN that is approximately stoichiometric, i.e., AlN thatcontains less than approximately 1% excess Al, less than approximately0.5% excess Al, or even less than approximately 0.1% excess Al. Afterformation, polycrystalline ceramic 195 may be stored in an inertatmosphere in preparation for utilization thereof to fabricatehigh-quality polycrystalline AlN source material.

While the polycrystalline AlN ceramic 195 may contain advantageously lowconcentrations of oxygen, various embodiments of the present inventionfeature a second stage of preparation that reduces or minimizesconcentrations of other contaminants such as carbon. In variousembodiments, this second stage involves an annealing and densificationtreatment of at least a portion of polycrystalline ceramic 195 to formhigh-quality polycrystalline AlN source material. In variousembodiments, the ceramic 195 is broken up into fragments before theannealing and densification treatment. The ceramic 195 may be fragmentedby, e.g., application of mechanical force, and one or more (typicallymore) of the fragments are collected and placed into a crucible (whichmay resemble crucible 130 or simply be a W vessel) for subsequent heattreatment. In various embodiments, larger fragments (e.g., ones havingwidths, diameters, or other lateral dimensions ranging from 0.5 cm to 2cm) are utilized while use of smaller particles or dust from ceramic 195(e.g., particles having large aggregate surface area) is avoided toavoid excess carbon contamination. For example, the fragments may beseparated on the basis of size using one or more sieves, and/orcompressed air or another fluid may be applied to the fragments tominimize or reduce the amount of dust or other particles thereon. Inother embodiments, substantially the entire ceramic 195 is placed intothe crucible and heat treated to form the polycrystalline AlN sourcematerial.

As shown in FIG. 1G, in accordance with various embodiments of theinvention, the ceramic 195 (or portion thereof) may be heated to a firsttemperature T1 ranging from 1100° C. to 1900° C. and held at temperatureT1 for a time period t1 of, for example, 2 hours to 25 hours.Thereafter, the ceramic 195 (or portion thereof) may be heated to ahigher second temperature T2 (e.g., a temperature ranging from 1900° C.to 2250° C.) and held at temperature T2 for a time period t2 of, forexample, 3 hours to 15 hours. During the heat treatment, the ceramic 195(or portion thereof) is annealed and densified to form a polycrystallineAlN source material that may be utilized in the subsequent formation ofhigh-quality single-crystal AlN bulk crystals. Because thepolycrystalline AlN source material is generally approximatelystoichiometric AlN with low concentrations of impurities, it may be usedto form an AlN bulk crystal without further preparation (e.g., withoutintermediate sublimation-recondensation steps).

FIG. 1H schematically depicts an alternative to the heat treatmentdepicted in FIG. 1G in which a longer ramp to temperature T2 is utilizedin place of the first annealing step at temperature T1. As shown in FIG.1H, in accordance with various embodiments of the invention, the ceramic195 (or portion thereof) may be ramped to temperature T2 (e.g., atemperature ranging from 1900° C. to 2250° C.) over a time period t1ranging from, for example, 5 hours to 25 hours. Thereafter, the ceramic195 (or portion thereof) may be held at temperature T2 for a time periodt2 of, for example, 3 hours to 25 hours. During the heat treatment, theceramic 195 (or portion thereof) is annealed and densified to form apolycrystalline AlN source material that may be utilized in thesubsequent formation of high-quality single-crystal AlN bulk crystals.Because the polycrystalline AlN source material is generallyapproximately stoichiometric AlN with low concentrations of impurities,it may be used to form an AlN bulk crystal without further preparation(e.g., without intermediate sublimation-recondensation steps).

In various embodiments, the oxygen concentration of the resultingpolycrystalline AlN source material, as measured by instrumental gasanalysis (IGA), ranges from approximately 1.0×10¹⁹ cm⁻³ to approximately3.0×10¹⁹ cm⁻³. In various embodiments, the carbon concentration of thepolycrystalline AlN source material, as measured by IGA, ranges fromapproximately 3.8×10¹⁸ cm⁻³ to approximately 1.8×10¹⁹ cm⁻³. The densityof the polycrystalline AlN source material, as measured by pycnometry atroom temperature, is approximately equal to that of single-crystal AlN,i.e., approximately 3.25 g/cm³ to 3.26 g/cm³. In contrast, the measureddensity of the AlN ceramic 195 is lower than that of the polycrystallineAlN source material, e.g., approximately 2.95 g/cm³ to approximately3.20 g/cm³. In addition, the polycrystalline AlN source materialtypically has an amber color and is composed of fairly large grains(e.g., average grain diameter ranging from approximately 0.1 mm toapproximately 5 mm).

FIG. 2 depicts a crystal-growth apparatus 200 suitable for the growth ofsingle-crystal AlN in accordance with various embodiments of the presentinvention. As shown, apparatus 200 includes a crucible 205 positioned ontop of a crucible stand 210 within a susceptor 215. Both the crucible205 and the susceptor 215 may have any suitable geometric shape, e.g.,cylindrical. During a typical growth process, an AlN boule 220 is formedby condensation of a vapor 225 that includes or consists essentially ofthe elemental precursors of the AlN boule 220, i.e., Al and N atomsand/or N₂ molecules. In typical embodiments, the vapor 225 arises fromthe sublimation of a source material 230, which may include, consistessentially of, or consist of the polycrystalline AlN source materialderived from ceramic 195 and described above. The AlN boule 220 may formon and extend from a seed crystal 235. (Alternatively, the AlN boule 220may nucleate upon and extend from a portion of the crucible 205 itself.)The seed crystal 235 may be a single crystal (e.g., a polished wafer)including, consisting essentially of, or consisting of AlN. In variousembodiments, the seed crystal 235 has a diameter (or width or otherlateral dimension) of at least approximately 35 mm, or even at leastapproximately 50 mm. In various embodiments, the seed crystal 235 has adiameter (or width or other lateral dimension) of approximately 150 mmor less, and single-crystal AlN grown therefrom has a diameter (or widthor other lateral dimension) of approximately 150 mm or less. In variousembodiments, the crystalline orientation of the seed crystal 235 issubstantially parallel to the c-axis. In other embodiments, thecrystalline orientation of the seed crystal 235 is at leastapproximately 5°, or even at least approximately 10° away from thec-axis; the orientation of the seed crystal 235 may be toward anon-polar direction.

The crucible 205 may include, consist essentially of, or consist of oneor more refractory materials, such as tungsten, rhenium, and/or tantalumnitride. As described in the '135 patent and the '153 patent, thecrucible 205 may have one or more surfaces (e.g., walls) configured toselectively permit the diffusion of nitrogen therethrough andselectively prevent the diffusion of aluminum therethrough.

In accordance with embodiments of the invention, one or more internalparts of the crystal-growth apparatus 200 (e.g., the crucible 205, thesusceptor 215, and/or the crucible stand 210) may be annealed beforecrystal growth and formation of AlN boule 220, and such annealing mayadvantageously decrease the carbon concentration (and/or the oxygenconcentration) in the AlN boule 220. In various embodiments, the one ormore internal parts of the crystal-growth apparatus 200 may be annealedat, for example, a temperature ranging from approximately 1000° C. toapproximately 1800° C. for a time period of approximately 5 hours toapproximately 50 hours.

In various embodiments of the invention, the concentration of oxygenand/or carbon within the AlN boule 220 may be decreased via theintroduction of one or more gettering materials within the crucible 205prior to and during growth of the AlN boule 220. The gettering materialsmay be introduced as a portion or all of one or more of the componentsof the crystal-growth apparatus 200 (e.g., the crucible 205, a linersituated within the crucible 205 and proximate an interior surface orwall thereof, the susceptor 215, and/or the crucible stand 210), and/orthe gettering materials may be introduced as discrete masses of materialwithin the crystal-growth apparatus 200. The gettering materials may bedisposed between the source material 230 and the growing AlN boule 220(but not, in various embodiments, in the direct line-of-sighttherebetween or blocking the entire direct line-of-sight therebetween)in order to, e.g., getter or absorb contaminants such as carbon and/oroxygen from the vapor flowing toward the AlN boule 220 (i.e., toward theseed crystal 235). In various embodiments, the gettering materials arestable at and have melting points greater than the growth temperature(e.g., greater than approximately 2000° C.) and have low vapor pressuresto prevent contamination of the growing AlN boule 220 with the getteringmaterials themselves. In various embodiments, a gettering material has aeutectic melting point with AlN that is greater than the growthtemperature (e.g., greater than approximately 2000° C.). Examples ofgettering materials in accordance with embodiments of the presentinvention include boron (melting point of approximately 2300° C.),iridium (melting point of approximately 2410° C.), niobium (meltingpoint of approximately 2468° C.), molybdenum (melting point ofapproximately 2617° C.), tantalum (melting point of approximately 2996°C.), rhenium (melting point of approximately 3180° C.), and/or tungsten(melting point of approximately 3410° C.). In various embodiments, thegettering material (or the component of the apparatus 200 or portionthereof) may include, consist essentially of, or consist of one or morenon-tungsten materials having melting temperatures of at leastapproximately 2300° C.

As shown in FIG. 2 , during formation of the AlN boule 220, apolycrystalline material 240 may form at one or more locations withinthe crucible 205 not covered by the seed crystal 235. However, thediameter (or other radial dimension) of the AlN boule 220 may expand,i.e., increase, during formation of the AlN boule 220, thereby occludingthe regions of polycrystalline material 240 from impinging vapor 225 andsubstantially limiting or even eliminating their growth. As shown inFIG. 2 , the diameter of the AlN boule 220 may expand to (or even startout at, in embodiments utilizing larger seed crystals 235) besubstantially equal to the inner diameter of the crucible 205 (in whichcase no further lateral expansion of the AlN boule 220 may occur).

The growth of the AlN boule 220 along a growth direction 245 typicallyproceeds due to a relatively large axial thermal gradient (e.g., rangingfrom approximately 5° C./cm to approximately 100° C./cm) formed withinthe crucible 205. A heating apparatus (not shown in FIG. 2 for clarity),e.g., an RF heater, one or more heating coils, and/or other heatingelements or furnaces, heats the susceptor 215 (and hence the crucible205) to an elevated temperature typically ranging between approximately1800° C. and approximately 2300° C. Prior to the onset of growth, thecrucible 205 and its contents (i.e., seed crystal 235, if present, andsource material 230) may be held at a temperature approximately equal tothe desired growth temperature for a predetermined soak time (e.g.,between approximately 1 hour and approximately 10 hours). In variousembodiments, this soak at temperature stabilizes the thermal fieldwithin the crucible 205, promotes effective nucleation on the seedcrystal 235, and promotes high-quality transition from nucleation tobulk growth of the single-crystalline AlN.

The apparatus 200 may feature one or more sets of top thermal shields250, and/or one or more sets of bottom axial thermal shields 255,arranged to create the large axial thermal gradient (by, e.g., betterinsulating the bottom end of crucible 205 and the source material 230from heat loss than the top end of crucible 205 and the growing AlNboule 220). During the growth process, the susceptor 215 (and hence thecrucible 205) may be translated within the heating zone created by theheating apparatus via a drive mechanism 260 in order to maintain theaxial thermal gradient near the surface of the growing AlN boule 220.One or more pyrometers 265 (or other characterization devices and/orsensors) may be utilized to monitor the temperature at one or morelocations within susceptor 215. The top thermal shields 250 and/or thebottom thermal shields 255 may include, consist essentially of, orconsist of one or more refractory materials (e.g., tungsten), and may bequite thin (e.g., between approximately 0.125 mm and 0.5 mm thick). Asdetailed in the '612 patent, the top thermal shields 250 and/or thebottom thermal shields 255 may be arranged in various configurationsand/or have various characteristics (i.e., different numbers of shields,different spacings between shields, different thicknesses, differentsized apertures defined therethrough, different sizes, etc.) in order toform a variety of different axial and radial thermal gradients withinthe crucible 205 and thus, the growth of the AlN boule 220 (e.g., thegrowth rate, the degree of radial expansion during growth, if any,etc.).

The maximum mass transfer from source material 230 and/or vapor 225 (andtherefore growth rate of AlN boule 220) is typically achieved bymaximizing the axial thermal gradient within the crucible 205 (i.e.,maximizing the temperature difference between the source material 230and the growing crystal 220 so that the growing crystal 220 has greatersupersaturation). In various embodiments, the onset of crystal-qualitydeterioration (e.g., increased dislocation density, formation of grainboundaries, and/or polycrystalline growth) sets the approximate upperlimit of the supersaturation at a given growth temperature. For typicalgrowth temperatures (e.g., between approximately 2125° C. andapproximately 2275° C.), this upper limit of the axial temperaturegradient is generally approximately 100° C./cm (although this maximummay depend at least in part on the dimensions and/or shape of the growthchamber, and may thus be larger for some systems). However, as thecross-sectional area of the AlN boule 220 increases (and/or forlarger-area seed crystals 235), the probability of parasitic nucleation(on the seed crystal 235 or in other locations) increases. Eachparasitic nucleation event may lead to formation of an additional growthcenter and result in grain or sub-grain formation (and thus low-qualityand/or polycrystalline material). Minimizing the probability ofparasitic nucleation is preferably achieved by providing a non-zeroradial thermal gradient in a direction substantially perpendicular tothe growth direction 245 that promotes lateral growth. Formation of theradial thermal gradient also enables growth of larger, high-qualitycrystals at high growth rates.

In various embodiments, the crucible 205 has a lid 270 with sufficientradiation transparency to enable at least partial control of the thermalprofile within the crucible 205 via the arrangement of the top thermalshields 250. Furthermore, in embodiments featuring a seed crystal 235,the seed crystal 235 is typically mounted on the lid 270 prior to thegrowth of AlN boule 220. The lid 270 is typically mechanically stable atthe growth temperature (e.g., up to approximately 2300° C.) and maysubstantially prevent diffusion of Al-containing vapor therethrough. Lid270 generally includes, consists essentially of, or consists of one ormore refractory materials (e.g., tungsten, rhenium, and/or tantalumnitride), and may be fairly thin (e.g., less than approximately 0.5 mmthick).

As shown in FIG. 2 , each of the top thermal shields typically has anopening 275 therethrough. The opening 275 normally echoes the geometryand/or symmetry of the crucible 205 (e.g., the opening 275 may besubstantially circular for a cylindrical crucible 205). The size of eachopening 275 may be varied; typically, the size(s) range from a minimumof 10 mm to a maximum of approximately 5 mm (or even 2 mm) less than thediameter of the crucible 205.

For example, in an embodiment, five thermal shields 250, each having adiameter of 68.5 mm and an opening size (diameter) of 45 mm, are used.The thickness of each of the thermal shields 250 is 0.125 mm, and thethermal shields 250 are spaced approximately 7 mm from each other. At atypical growth temperature of 2065° C., this shield arrangement resultsin a radial thermal gradient (measured from the center of thesemiconductor crystal to the inner edge of the crucible) of 27° C./cm.

As shown in FIG. 2 , the top thermal shields 250 may have openings 275larger than any such opening present in the bottom thermal shields 255,and/or the top thermal shields 250 may be stacked with one or morespacings between shields that are larger than that between the variousbottom thermal shields 255. The spacings may range between approximately1 mm and approximately 20 mm, or even between approximately 7 mm andapproximately 20 mm. Also as shown, the openings 275 in the top thermalshields 250 may all be substantially equal to each other. Depending uponthe desired growth conditions (e.g., pressure, temperature, crucibledimensions, distance between the seed crystal and the source material,etc.), the number of top thermal shields 250, the spacing betweenshields 250, and/or the size of the openings 275 may be varied to formthe desired radial thermal gradient. The radial thermal gradient mayeven be varied in real time during the growth of AlN boule 220, e.g., inresponse to feedback based on determination of the radial thermalgradient during growth. For example, the radial thermal gradient may bedetermined based on the temperatures of lid 270 and one or more sides ofcrucible 215 (e.g., measured by pyrometers 265 as shown in FIG. 2 ).

Similarly, although each of the top thermal shields 250 may have athickness less than 0.5 mm, the thickness of one or more of the shields250 may be varied with respect to the others. For example, one or moretop thermal shields 250 may have a thickness of approximately 0.25 mmwhile one or more others have a thickness of approximately 0.125 mm. Thethickness of the top thermal shields 250 may even be varied as afunction of distance away from the lid 270 (i.e., either increasing ordecreasing). Such thermal shields 250 having different thicknesses maybe utilized to adjust the thermal field above and within the crucible215. For example, a thicker shield may be used to block more radiativeheat flow but will typically have higher thermal impact at temperatureswhere the heat flux is dominated by the thermal conductivity (lowertemperatures, e.g. <1500°-1800°). Therefore, the resultant radialthermal gradient may vary as a function of growth temperature, even withthe same arrangement of the same top thermal shields 250.

After growth of the AlN boule 220, controlled cooling techniques may beutilized to maintain, and even enhance in many embodiments, the UVtransparency of the AlN boule 220 as it is cooled to room temperature.FIG. 3 schematically depicts two different cooling cycles that may beutilized in accordance with embodiments of the present invention. Asshown, in a first embodiment (“Path 1”), the AlN boule 220 undergoes aslow-cooling stage followed by a fast-cooling stage. Specifically, theAlN boule 220 may be first cooled from the growth temperature to anintermediate temperature T_(A) ranging from approximately 1450° C. toapproximately 2150° C. over a time tai ranging from approximately 10minutes to approximately 90 minutes. After reaching temperature T_(A),the AlN boule 220 may be cooled to a second intermediate temperatureT_(B) ranging from approximately 1000° C. to approximately 1650° C. overa time t_(B1) ranging from approximately 10 seconds to approximately 10minutes. After reaching temperature T_(B), the AlN boule 220 may be leftto cool to approximately room temperature (e.g., approximately 25° C.)at an uncontrolled rate, i.e., a rate depending only on the cooling rateof the growth system (i.e., the surroundings of AlN boule 220) withoutpower applied to the heating elements thereof. At any point aftercooling to temperature T_(B), the AlN boule 220 and the crucible 205 maybe removed from the growth system to cool in the surrounding ambient toapproximately room temperature.

As also shown in FIG. 3 , in a second embodiment (“Path 2”), the AlNboule 220 undergoes a fast-cooling stage followed by a slow-coolingstage. Specifically, the AlN boule 220 may be first cooled from thegrowth temperature to the intermediate temperature T_(A) over a timet_(A2) ranging from approximately 10 seconds to approximately 10minutes. After reaching temperature T_(A), the AlN boule 220 may becooled to the second intermediate temperature T_(B) over a time t_(B2)ranging from approximately 10 minutes to approximately 90 minutes. Afterreaching temperature T_(B), the AlN boule 220 may be left to cool toapproximately room temperature (e.g., approximately 25° C.) at anuncontrolled rate, i.e., a rate depending only on the cooling rate ofthe growth system (i.e., the surroundings of AlN boule 220) withoutpower applied to the heating elements thereof. At any point aftercooling to temperature T_(B), the AlN boule 220 and the crucible 205 maybe removed from the growth system to cool in the surrounding ambient toapproximately room temperature.

During the cooling of the AlN boule 220 to intermediate temperaturesT_(A) and T_(B), the temperature of the growth system surrounding thecrucible 205 may be controlled to obtain the desired temperature changesover the desired times. For example, the power supplied to heatingelements (e.g., RF coil) may be decreased over a desired time to adjustthe resulting temperature of the AlN boule 220, and/or the temperaturemay be directly controlled via feedback enabled by temperaturemeasurements from, e.g., pyrometers or other temperature sensors in thegrowth system. While the temperature changes along Paths 1 and 2 in FIG.3 are depicted as linear, in various embodiments these changes may havean exponential dependence, particularly in embodiments in which apower-controlled cool-down is applied.

In various embodiments, the choice between Paths 1 and 2 may be madedepending on various parameters of the growth apparatus. For example, ina setup in which the AlN boule 220 is strongly bonded to itssurroundings (e.g., lid 270, crucible 205, etc.), there may be a higherprobability of stress resulting from thermal-expansion mismatch betweenthe AlN boule 220 and those surroundings, which may result in crackingand/or an increased dislocation density in AlN boule 220. Such strongbonding may result from large thermal gradients (e.g., axial and/orlateral) within the crucible 205, and the bond strength may increase asthose thermal gradients increase. In such cases, Path 2 may bepreferred. In embodiments in which the AlN boule 220 is only looselybonded to its surroundings, and/or in which the thermal-expansioncoefficients of the AlN boule 220 and the crucible 205 are similar, Path1 may be selected. Moreover, the present inventors have cooled AlNboules 220 via both Paths 1 and 2, and the UV absorption coefficients ofthe resulting crystals are generally lower than in similarly growncrystals that are cooled down from the growth temperature to atemperature of approximately 1000° C. in a single rapid step (e.g., overapproximately 10 minutes). In various embodiments, the two-stage coolingcycles described above (i.e., Paths 1 and 2) may be replaced with asingle-stage cooling cycle that maintains, and even enhances in manyembodiments, the UV transparency of the AlN boule 220 as it is cooled toroom temperature. In single-stage cooling cycles in accordance withembodiments of the invention, the fast-cooling stage of either Path 1 or2 may be eliminated (i.e., have a time of approximately zero), and theAlN boule 220 may be first cooled from the growth temperature to theintermediate temperature T_(B) ranging from approximately 1000° C. toapproximately 1650° C. over a time period ranging from approximately 1hour to approximately 10 hours. After reaching temperature T_(B), theAlN boule 220 may be left to cool to approximately room temperature(e.g., approximately 25° C.) at an uncontrolled rate, i.e., a ratedepending only on the cooling rate of the growth system (i.e., thesurroundings of AlN boule 220) without power applied to the heatingelements thereof. At any point after cooling to temperature T_(B), theAlN boule 220 and the crucible 205 may be removed from the growth systemto cool in the surrounding ambient to approximately room temperature.

In various embodiments of the present invention, the two-stage orsingle-stage cool-down cycles described in reference to FIG. 3 may beutilized after an annealing cycle to enhance UV transparency ofpreviously grown AlN bulk crystals (e.g., boules or substrates separatedtherefrom). For example, in various embodiments, an AlN bulk crystal(grown in accordance with embodiments of the present invention or viaother techniques) may be annealed at a temperature exceeding 2000° C.,e.g., ranging from approximately 2000° C. to approximately 2400° C.Thereafter, the AlN bulk crystal may be cooled down to room temperaturevia Path 1 or Path 2 as detailed above, or via the single-stage coolingcycle detailed above. The use of such an annealing and cool-down cyclemay advantageously increase the UV transparency (e.g., decrease the UVabsorption coefficient for one or more wavelengths) of the AlN bulkcrystal.

After formation of AlN boule 220, one or more substrates (or “wafers”)may be separated from AlN boule 220 by the use of, e.g., a diamondannular saw or a wire saw. In an embodiment, a crystalline orientationof a substrate thus formed may be within approximately 2° (or evenwithin approximately 1°, or within approximately 0.5°) of the (0001)face (i.e., the c-face). Such c-face wafers may have an Al-polaritysurface or an N-polarity surface, and may subsequently be prepared asdescribed in U.S. Pat. No. 7,037,838, the entire disclosure of which ishereby incorporated by reference. In other embodiments, the substratemay be oriented within approximately 2° of an m-face or a-faceorientation (thus having a non-polar orientation) or may have asemi-polar orientation if AlN boule 220 is cut along a differentdirection. The surfaces of these wafers may also be prepared asdescribed in U.S. Pat. No. 7,037,838. The substrate may have a roughlycircular cross-sectional area with a diameter of greater thanapproximately 50 mm. The substrate may have a thickness that is greaterthan approximately 100 μm, greater than approximately 200 μm, or evengreater than approximately 2 mm. The substrate typically has theproperties of AlN boule 220, as described herein. After the substratehas been cut from the AlN boule 220, one or more epitaxial semiconductorlayers and/or one or more light-emitting devices, e.g., UV-emittinglight-emitting diodes or lasers, may be fabricated over the substrate,for example as described in U.S. Pat. Nos. 8,080,833 and 9,437,430, theentire disclosure of each of which is hereby incorporated by reference.

FIGS. 4A-4E depict various characteristics of crack-free AlN bulkcrystals having diameters of approximately 50 mm produced in accordancewith embodiments of the present invention. As shown in FIG. 4A, suchcrystals have high UV transparencies, e.g., UV absorption coefficientsof less than 30 cm⁻¹ for the UV wavelengths of approximately 220 nm toapproximately 480 nm, and less than 20 cm⁻¹ for wavelengths ofapproximately 250 nm to approximately 480 nm. FIG. 4B is a micrograph ofan etch pit density measurement (i.e., an etching measurement thatreveals defects such as threading dislocations intersecting the surfaceof the crystal) of approximately 7×10³ cm⁻². FIG. 4C is an x-raytopography map of an AlN crystal grown in accordance with embodiments ofthe invention revealing a density of threading edge dislocations ofapproximately 3×10³ cm⁻² and a density of threading screw dislocationsof approximately 10 cm⁻², i.e., a total threading dislocation densityless than approximately 10⁴ cm⁻². FIGS. 4D and 4E depict x-ray rockingcurves (along (0002) and (10-12) respectively) having full width at halfmaximum (FWHM) values less than 50 arcsec. As measured by secondary ionmass spectroscopy (SIMS), the samples depicted in FIGS. 4A-4E had carbonconcentrations less than 3×10¹⁷ cm⁻³-4×10¹⁷ cm⁻³, oxygen concentrationsless than 1×10¹⁷ cm⁻³-4×10¹⁷ cm⁻³, silicon concentrations less than1×10¹⁷ cm⁻³, and ratios of carbon to oxygen concentration of less than0.5.

In various embodiments of the invention, oxygen may be intentionallyadded to the AlN crystal during and/or after growth in order to maintainthe ratio of carbon to oxygen concentration in the AlN crystal at alevel of less than 0.5. As detailed herein, various measures may betaken to minimize the carbon concentration within the AlN crystal (e.g.,to a level of approximately 3×10¹⁷ cm⁻³ or less), and thus, in theabsence of additional oxygen, the ratio of carbon to oxygen within thecrystal may be greater than 0.5. Thus, oxygen gas may be introduced intothe growth crucible during at least a portion of the growth of the AlNcrystal, and/or the AlN crystal may be annealed in an oxygen-containingambient after growth. In various embodiments, the polycrystalline AlNsource material may be exposed to oxygen (e.g., within anelevated-temperature anneal cycle), and the oxygen thus absorbed intothe source material may be released into the vapor phase during growth.While the present inventors do not wish to be bound by any particulartheory of operation of such additional oxygen, the introduction ofadditional oxygen in the AlN crystal may have one or more beneficialeffects resulting in increased UV transparency. For example, the oxygenmay react with any carbon within the vapor phase to form CO and/or CO₂,as carbon has a low vapor pressure and is typically mainly transportedby attaching itself to another species or being flushed toward thecrystal due to a high flux of vapor. The additional oxygen may alsocreate point defects (e.g., vacancies and/or complexes) within the AlNcrystal that reduce the UV light absorption centers resulting fromcarbon impurities (e.g., via vacancy annihilation).

In various embodiments, as measured by SIMS, AlN single crystals havingdiameters of at least 50 mm may have carbon concentrations ofapproximately 0.6×10¹⁷ cm⁻³-6.2×10¹⁷ cm⁻³, as well as oxygenconcentrations of approximately 1×10¹⁷ cm⁻³-7.9×10¹⁷ cm⁻³, The thermalconductivity of AlN boule 220 and/or substrates derived therefrom may begreater than approximately 290 Watts per meter-Kelvin (W/m·K), asmeasured by the American Society for Testing and Materials (ASTM)Standard E1461-13 (Standard Test Method for Thermal Diffusivity by theFlash Method), the entire disclosure of which is incorporated byreference herein, and provided by a commercial vendor such as NETZSCHInc. of Exton, Pa. As shown in FIG. 4F, the thermal conductivity of acrack-free AlN bulk crystal having a diameter of approximately 50 mmproduced in accordance with embodiments of the present invention wasapproximately 293 W/m·K with a slope of −1.55. In this sample, theoxygen and carbon concentration were confirmed by SIMS as 3.5×10¹⁷ cm⁻³and 1.2×10¹⁷ cm⁻³, respectively.

The present inventors have also found that there is a strong correlationbetween the visible photoluminescence (PL) color and the carbonconcentration in single-crystal AlN produced in accordance withembodiments of the present invention. Specifically, in variousembodiments, an AlN single crystal may be illuminated with a mercurylight source having a wavelength of 254 nm. The resulting luminescencemay be observed with the naked eye or captured with an imaging devicesuch as a digital camera. If the resulting luminescence is bright blue,it corresponds to high carbon concentration and high resulting UVabsorption, whereas dark blue, black, or dark green luminescencecorresponds to low carbon concentration. FIG. 5A is a series ofmicrographs of various single-crystal AlN wafers illuminated undernormal lighting conditions, as well as the PL luminescence of thesamples when illuminated as described above. Areas on the substratesthat exhibited bright blue luminescence were also found to have highlevels of carbon and corresponding high levels of UV absorption,particularly at wavelengths near 265 nm. The corresponding impurityconcentrations in the sampled areas (indicated by the small circles), aswell as the UV absorption coefficients at 265 nm, for the samples ofFIG. 5A are shown in the table of FIG. 5B, where sample (1) correspondsto the top sample of FIG. 5A, and the sample numbers increase downwardin numerical order. As shown in FIGS. 5A and 5B, not only does the PLluminescence correlate well to UV absorption and impurity concentration,but the UV absorption coefficient itself correlates well with carbonconcentration and the ratio of carbon to oxygen in the samples.

FIGS. 6A and 6B illustrate the relationship between UV absorptioncoefficient for 50 mm-diameter AlN single crystals at a wavelength of265 nm and the carbon concentration (FIG. 6A) and the ratio of carbonconcentration to oxygen concentration (FIG. 6B) measured via SIMS. Asshown, the UV absorption coefficient is quite strongly related to boththe carbon concentration and the carbon-to-oxygen ratio, and thatembodiments of the invention having UV absorption coefficients of lessthan 50 cm⁻¹ have carbon concentrations less than about 3×10¹⁷ cm⁻³ andcarbon-to-oxygen concentration ratios of less than about 0.5.

AlN bulk crystals produced in accordance with embodiments of the presentinvention advantageously exhibit large ultraviolet (UV) transparencymetrics, where the UV transparency metric is defined in cm³ as:

$\frac{d}{10 \times FWHM \times a^{2}}$

where d is the diameter of the AlN crystal in mm, FWHM is the full-widthat half-maximum of an x-ray rocking curve of the AlN crystal in radians,and a is the absorption coefficient of the AlN crystal at the wavelengthof interest. For example, AlN bulk crystals may have UV transparencymetrics ranging from approximately 5 cm³ to approximately 5000 cm³ at awavelength of 265 nm, or even ranging from approximately 30 cm³ toapproximately 5000 cm³ at a wavelength of 265 nm. Such AlN crystals mayhave diameters (or widths or other lateral dimensions) of at leastapproximately 50 mm. The table below depicts various UV transparencymetrics as a function of crystal diameter, FWHM, and absorptioncoefficient.

Crystal Diameter FWHM Abs. Coeff. UV Trans. Metric (inch) (arcsec)(cm⁻¹) (cm³) 1 50 45 5 1 25 30 23 1 10 1 52393 2 300 80 1 2 50 45 10 235 45 15 2 50 30 23 2 35 30 33 2 50 5 838 2 10 5 4191 2 10 1 104785

The table below depicts ranges of the UV transparency metric for AlNsingle crystals produced in accordance with embodiments of the inventionand having FWHM of 25 arcsec as a function of wavelength and diametersof at least 50 mm.

Wavelength (nm) UV Trans. Metric (cm³) 210 1-11 220 5-47 230 5-95 240 5-116 250  5-129 255  5-145 265  5-186 280 10-186 310 10-291 350 20-517410 20-855 450  50-1677 500  50-2620

In accordance with embodiments of the present invention, the outputpower of UV-emitting light-emitting devices such as LEDs isadvantageously increased as the UV transparency metric of the underlyingsubstrate increases. In an example, six AlN single-crystal substrateswere cut from a boule produced as described herein but without thecontrolled cool-down cycle detailed above with respect to FIG. 3 . Sixadditional AlN single-crystal substrates were cut from a boule producedsimilarly but with the controlled cool-down cycle. All substrates haddiameters of approximately 50 mm. UV LEDs emitting at 265 nm werefabricated on all 12 substrates, and the device power from the devicesat an operating power of 100 mA was measured without any thinning orremoval of the underlying AlN substrate. The UV absorption of eachsubstrate was measured for the wavelength of 265 nm and averaged over 52different locations over each substrate. The FWHM of the x-ray rockingcurves for each substrate was the average of 13 different measurementseach at a different location. FIG. 7 is a graph of the output power as afunction of the UV transparency metric and, as shown, the output powerexhibits a strong dependence on the UV transparency metric. Thus, the UVtransparency metric as defined herein provides a suitable tool forestimation of light-emitting device performance for devices ultimatelyfabricated on AlN single-crystal substrates produced in accordance withembodiments of the present invention.

Single-crystal AlN, and wafers formed therefrom, may be utilized forfabrication of electronic and optoelectronic devices thereon. Forexample, portions of AlN single crystals grown in accordance withembodiments of the invention detailed herein may be utilized assubstrates for subsequent epitaxial growth and processing for theformation of LEDs and/or lasers that emit light in the ultravioletwavelength range.

FIG. 8A schematically depicts a light-emitting device structure 800 inaccordance with embodiments of the present invention. Light-emittingdevice structures 800 in accordance with embodiments of the inventionmay include, consist essentially of, or consist of, for example,light-emitting diodes or lasers. As shown, the device structure 800includes a substrate 805, which in various embodiments includes,consists essentially of, or consists of aluminum nitride, e.g.,single-crystal aluminum nitride. In various embodiments, substrate 805is a single-crystal substrate separated from single-crystalline AlNgrown in accordance with embodiments of the invention after formationthereof. In various embodiments, the substrate 805 may not betransparent (at least not fully transparent) to radiation emitted by thedevice structure 800 (e.g., UV radiation), depending upon thewavelength(s) emitted by device structure 800. Substrate 805 may bemiscut such that the angle between its c-axis and its surface normal isbetween approximately 0° and approximately 4°. In various embodiments,the misorientation of the surface of substrate 805 is less thanapproximately 0.3°, e.g., for substrates 805 that are not deliberatelyor controllably miscut. In other embodiments, the misorientation of thesurface of substrate 805 is greater than approximately 0.3°, e.g., forsubstrates 805 that are deliberately and controllably miscut. In variousembodiments, the direction of the miscut is towards the a-axis.

The surface of substrate 805 may have a group-III (e.g., Al—) polarity,and may be planarized, e.g., by chemical-mechanical polishing. The RMSsurface roughness of substrate 805 may be less than approximately 0.5 nmfor a 10 μm×10 μm area. In some embodiments, atomic-level steps aredetectable on the surface when probed with an atomic-force microscope.The threading dislocation density of substrate 805 may be measuredusing, e.g., etch pit density measurements after a 5 minute KOH—NaOHeutectic etch at 450° C. In various embodiments, the threadingdislocation density is less than approximately 2×10³ cm⁻². In someembodiments substrate 805 has an even lower threading dislocationdensity. Substrate 805 may be topped with a homoepitaxial layer (notshown) that may include, consist essentially of, or consist of doped orundoped AlN.

The various layers of device structure 800 disposed over substrate 805may be formed by any of a variety of different techniques, e.g.,epitaxial growth techniques such as chemical vapor deposition (CVD)methods such as metallorganic CVD (MOCVD).

In accordance with embodiments of the invention, a release layer 810 maybe disposed over the substrate 805 to facilitate later removal of all ora portion of the substrate 805 from the rest of device structure 800,for example as described in U.S. patent application Ser. No. 15/977,031,filed on May 11, 2018 (the '031 patent application), the entiredisclosure of which is incorporated by reference herein. In variousembodiments, the release layer 810 is substantially lattice-matched tothe substrate 805. Minimizing the lattice mismatch between the releaselayer 810 and the substrate 805 advantageously reduces or eliminates,for example, cracking and/or defect introduction in the release layer810 and island formation (i.e., three-dimensional growth) during growthof the release layer 810. (As used herein, a layer that is“substantially lattice-matched” to a substrate or another layer has anunstrained lattice parameter sufficiently close to that of the substrateor other layer to enable epitaxial growth of the layer thereover suchthat the layer is approximately lattice-relaxed, or tensilely orcompressively strained without significant strain relaxation (e.g., lessthan 20% relaxation, or even less than 10% relaxation), and/or to enableepitaxial growth of the layer without introduction of cracks and/ordefects (e.g., dislocations) at densities exceeding those, if any,present in the underlying substrate or layer.) In various embodiments,the lattice mismatch between the release layer 810 and the substrate 805is less than ±5%, less than ±3%, less than ±2%, less than ±1%, less than±0.5%, less than ±0.2%, or less than ±0.1%. In various embodiments, itmay be preferable to reduce the lattice mismatch when the release layer810 is tensilely strained (i.e., the unstrained, innate in-plane latticespacing of the release layer 810 is smaller than that of substrate 805)in order to minimize or eliminate stress-relieving cracking in therelease layer 810. In various embodiments, when the release layer 810 iscompressively strained, the lattice mismatch to the substrate 805 may belarger but may be a function of the thickness of release layer 810. Forexample, compressively strained release layers 810 having too muchlattice mismatch to the substrate 805 and too large a thickness mayisland during layer growth. Thus, in various embodiments, a releaselayer 810 having a compressive lattice mismatch with substrate 805 ofapproximately 3% may have a thickness no more than approximately 10 nm.For layers with less lattice mismatch, the thickness may be larger.

In various embodiments, the release layer 810 includes, consistsessentially of, or consists of AlN or AlGaN doped with one or moreimpurities that form an absorption band within the release layer 810 fora wavelength of light not strongly absorbed by the substrate 805 itself.For example, the release layer 810 may include, consist essentially of,or consist of AlN doped with oxygen, which has an absorption band atapproximately 310 nm. Since an AlN substrate 805 does not stronglyabsorb light having wavelengths larger than approximately 300 nm,absorption of light within, and concomitant local heating of, therelease layer 810 may be utilized to remove the substrate 805 from thedevice structure 800. In various embodiments, the release layer 810 maybe doped with oxygen (O) and/or one or more other dopants, for example,carbon (C), iron (Fe), manganese (Mn), or gadolinium (Gd). Such dopantsmay be introduced (e.g., as an additional gaseous species) duringepitaxial growth of the release layer 810. In other embodiments, some orall of the dopant may be introduced after epitaxial growth of at least aportion of the release layer 810 by techniques such as ion implantationor dopant diffusion (e.g., from a solid or gaseous source). In variousembodiments of the invention, one or more of the dopants may beintroduced into and/or present within the release layer 810 at aconcentration of at least for example, approximately 10¹⁷ cm⁻³, at leastapproximately 10¹⁸ cm⁻³, or even at least 10¹⁹ cm⁻³. In variousembodiments of the invention, one or more of the dopants may beintroduced into and/or present within the release layer 810 at aconcentration of at most for example, approximately 10²⁰ cm⁻³, or atmost approximately 10²¹ cm⁻³.

In exemplary embodiments, a release layer 810 including, consistingessentially, or consisting of AlN doped with oxygen may exhibit anabsorption band at a wavelength of approximately 310 nm, and a releaselayer 810 including, consisting essentially, or consisting of AlN dopedwith carbon may exhibit an absorption band at a wavelength ofapproximately 265 nm. In such embodiments, radiation for substrateseparation may be applied via, for example, a KrF laser (emissionwavelength of approximately 248 nm) or a XeCl laser (emission wavelengthof approximately 308 nm).

In various embodiments, release layer 810 may include, consistessentially of, or consist of a semiconductor other than AlN (e.g.,AlGaN), and which may contain one or more dopants forming one or moreabsorption bands for light that is not strongly absorbed by substrate805. In various embodiments, the release layer 810 may include, consistessentially of, or consist of a nitride alloy containing one or more ofboron, aluminum, gallium, and/or indium. The release layer 810 may eveninclude, consist essentially of, or consist of silicon carbide or ametal nitride (in which the metal is, e.g., one or more of Sc, Y, La,Ti, or Ta). For example, a release layer 810 including, consistingessentially, or consisting of silicon carbide may exhibit an absorptionband at a wavelength of approximately 376 nm, and a release layer 810including, consisting essentially, or consisting of titanium nitride mayexhibit an absorption band at a wavelength of approximately 365 nm. Invarious embodiments, the release layer 810 is substantiallylattice-matched to substrate 805.

In various embodiments, multiple release layers 810 may be presentwithin device structure 300, and each release layer 810 may have one ormore absorption bands different from one or all absorption bands in theother release layer(s) 810. For example, multiple release layers 810including, consisting essentially of, or consisting of AlN or AlGaN maybe formed (e.g., epitaxially grown), where each release layer 810 isdoped with a different one of the dopants referred to above. In variousembodiments, one or more release layers 810 may be tensilely strainedwith respect to the substrate 805, and/or one or more release layers 810may be compressively strained with respect to the substrate 805. Invarious embodiments of the invention, release layer 810 is not presentwithin the device structure 800.

Device structure 800 also includes an active light-emitting structure815 disposed over the release layer 810, if the release layer 810 ispresent, as shown in FIG. 8A. For example, the active structure 815 mayinclude a bottom contact layer 820. In various embodiments, the bottomcontact layer 820 is n-type doped, e.g., doped with an impurity such asP, As, Sb, C, H, F, O, Mg, and/or Si. The bottom contact layer 820 mayinclude, consist essentially of, or consist of, for example, AlN orAl_(x)Ga_(1-x)N. In an embodiment, an optional graded buffer layer (notshown) is disposed above substrate 805 and below bottom contact layer820 (and, in various embodiments, below the release layer 810, if therelease layer 810 is present). The graded buffer layer may include,consist essentially of, or consist of one or more semiconductormaterials, e.g., Al_(x)Ga_(1-x)N. In various embodiments, the gradedbuffer layer has a composition approximately equal to that of substrate805 at the bottom interface of the graded buffer layer in order topromote two-dimensional growth and avoid deleterious islanding (suchislanding may result in undesired elastic strain relief and/or surfaceroughening in the graded buffer layer and subsequently grown layers).The composition of the graded buffer layer at an interface with bottomcontact layer 820 (or release layer 810, if present) may be chosen to beclose to (e.g., approximately equal to) that of the desired activeregion of the device (e.g., the Al_(x)Ga_(1-x)N concentration that willresult in the desired wavelength emission from the light-emittingdevice). In an embodiment, the graded buffer layer includes, consistsessentially of, or consists of doped or undoped Al_(x)Ga_(1-x)N gradedfrom an Al concentration x of approximately 100% to an Al concentrationx ranging from approximately 60% to approximately 85%.

The bottom contact layer 820 may have a thickness sufficient to preventcurrent crowding after device fabrication and/or to stop on duringetching to fabricate contacts. For example, the thickness of bottomcontact layer 820 may range from approximately 100 nm to approximately500 nm, or from approximately 100 nm to approximately 2 μm. Whenutilizing a bottom contact layer 820, the final light-emitting devicemay be fabricated with back-side contacts. In various embodiments,bottom contact layer 820 will have high electrical conductivity evenwith a small thickness due to the low defect density maintained when thelayer is pseudomorphic. As utilized herein, a pseudomorphic film is onewhere the strain parallel to the interface between the film and anunderlying layer or substrate is approximately that needed to distortthe lattice in the film to match that of the substrate (or a relaxed,i.e., substantially unstrained, layer over the substrate and below thepseudomorphic film). Thus, the parallel strain in a pseudomorphic filmwill be nearly or approximately equal to the difference in latticeparameters between an unstrained substrate parallel to the interface andan unstrained epitaxial layer parallel to the interface.

Active structure 815 is configured for the emission of light in responseto an applied voltage. Thus, the active structure 815 may include amultiple-quantum well (“MQW”) layer 825 disposed above bottom contactlayer 820. In various embodiments, MQW layer 825 is disposed directly onthe bottom contact layer 820. In other embodiments, an optional layer(e.g., an undoped layer including, consisting essentially of, orconsisting of an undoped semiconductor material such as AlGaN) may bedisposed between the bottom contact layer 820 and the MQW layer 825. TheMQW layer 825 may be doped with the same doping polarity as the bottomcontact layer 820, e.g., n-type doped. The MQW layer 825 may include,consist essentially of, or consist of one or more quantum wellsseparated by (or surrounded on both sides by) barriers. For example,each period of MQW layer 825 may feature an Al_(x)Ga_(1-x)N quantum welland an Al_(y)Ga_(1-y)N barrier, where x is different from y. Typically,y is greater than 0.4 for light-emitting devices designed to emit lighthaving a wavelength less than 300 nm and may be greater than 0.7 forshorter-wavelength emitters. It may even be greater than 0.9 for devicesdesigned to emit at wavelengths shorter than 250 nm. The value of xwill, at least in part, determine the emission wavelength of the device.For emission wavelengths longer than 280 nm, x may be as low as 0.2. Forwavelengths between 250 nm and 280 nm, x may vary between 0.2 and 0.7.For wavelengths shorter than 250 nm, x may be greater than 0.6. Invarious embodiments, the difference between x and y is large enough toobtain good confinement of the electrons and holes in the active region,thus enabling high ratio of radiative recombination to non-radiativerecombination. In an embodiment, the difference between x and y isapproximately 0.25, e.g., x is approximately 0.5 and y is approximately0.75. MQW layer 825 may include a plurality of such periods, and mayhave a total thickness ranging from 20 nm to 100 nm, or less thanapproximately 50 nm. In various embodiments of the invention, the activelight-emitting structure 815 is configured to (e.g., has a MQW layer 825having layer composition(s) selected to) emit ultraviolet light.

In various embodiments of the invention, an electron-blocking layer 830may be disposed over MQW layer 825. The electron-blocking layer 830typically has a wider band gap than that of a band gap within the MQWlayer 825 (e.g., a band gap of the barrier layers therewithin). Invarious embodiments, the electron-blocking layer 830 may include,consist essentially of, or consist of e.g., Al_(x)Ga_(1-x)N, andelectron-blocking layer 830 may be doped. For example, theelectron-blocking layer 830 may be doped with the same doping polarityas that of bottom contact layer 820 and/or MQW layer 825 (e.g., n-typedoped). In various embodiments, the value of x in the electron-blockinglayer 830 is greater than the value of the Al mole fraction in thebarrier layers used in the MQW layer 825. For longer wavelength deviceswith emission wavelengths greater than 300 nm, x may be as low as 0.4and may be greater than 0.7 for shorter wavelength devices. It may evenbe greater than 0.9 for devices designed to emit at wavelengths shorterthan 250 nm. Electron-blocking layer 830 may have a thickness that mayrange, for example, between approximately 10 nm and approximately 50 nm,or even between approximately 10 nm and approximately 30 nm. In variousembodiments of the invention, the electron-blocking layer 830 issufficiently thin (e.g., thickness less than about 30 nm, or less thanabout 20 nm) so as to facilitate carrier (e.g., hole) tunneling throughthe electron-blocking layer 830. In various embodiments of theinvention, the electron-blocking layer 830 is omitted from devicestructure 800.

As shown in FIG. 8A, device structure 800 may also include a gradedlayer 835 disposed above the electron-blocking layer 830 (or above theMQW layer 825 in embodiments in which electron-blocking layer 830 isomitted), and a cap layer (or “top contact layer”) 840 may be disposedover the graded layer 835. The cap layer 840 may be doped with a dopingpolarity opposite of that of the bottom contact layer 820, e.g., p-typedoped with one or more dopants such as Mg, Be, and/or Zn. In otherembodiments, the cap layer 840 may be undoped, as carriers (e.g., holes)may be injected from an electrode into a two-dimensional carrier gasdisposed at the interface between the cap layer 840 and the graded layer835. (While in exemplary embodiments described herein the cap layer 840is doped p-type and the bottom contact layer 820 is doped n-type,embodiments in which the doping polarities of these layers are switchedare within the scope of the present invention; in such embodiments, theelectron-blocking layer 830, if present, may be considered to be a“hole-blocking layer,” as understood by those of skill in the art.) Thecap layer 840 may have a thickness ranging from, e.g., approximately 1nm to approximately 100 nm, or approximately 1 nm to approximately 50nm, or approximately 1 nm to approximately 20 nm. In variousembodiments, the cap layer 840 includes, consists essentially of, orconsists of Al_(x)Ga_(1-x)N, and in various embodiments the aluminumconcentration x may range from 0 (i.e., pure GaN) to approximately 0.2.

The device structure 800 may also incorporate one or more metalliccontacts to facilitate electrical contact to the device. For example,one metallic contact may include or consist essentially of an electrodelayer 845 disposed above or on the cap layer 840. The composition and/orshape of the electrode layer 845 are not particularly limited as long asit enables the injection of carriers (e.g., holes) into the cap layer840. In embodiments in which holes are injected into a p-type dopednitride-based semiconductor cap layer 840, the electrode layer 845 mayinclude, consist essentially of, or consist of one or more metals havinglarge work functions, e.g., Ni, Au, Pt, Ag, Rh, and/or Pd, alloys ormixtures of two or more of these metals, or oxide-based and/ortransparent electrode materials such as indium tin oxide (ITO). Inembodiments in which electrons are injected into an n-type dopednitride-based semiconductor cap layer 840, the electrode layer 845 mayinclude, consist essentially of, or consist of one or more metals, e.g.,Ti, Al, Au, Pt, Ni, and/or V, alloys or mixtures of two or more of thesemetals, or oxide-based and/or transparent electrode materials such asindium tin oxide (ITO). Electrode layers 845 in accordance withembodiments of the invention are not limited to these materials. Thethickness of the electrode layer 845 may be, for example, betweenapproximately 10 nm and approximately 100 nm, or between approximately10 nm and approximately 50 nm, or between approximately 10 nm andapproximately 30 nm, or between approximately 25 nm and approximately 40nm. In various embodiments, the electrode layer 845 is formed afterremoval of all or a portion of the substrate 805.

In various embodiments, a second electrode layer 850 may be formed inelectrical contact with (and, in various embodiments, direct mechanicalcontact with) the bottom contact layer 820, even if the substrate 805 isnot removed, as shown in FIG. 8B; such an electrode layer 850 may beconsidered to be a “bottom electrode layer.” For example, a portion ofthe active structure 815 may be removed by, e.g., conventionalphotolithography and wet and/or dry etching, in order to reveal at leasta portion of the bottom contact layer 820. The second electrode layer850 may then be deposited on the bottom contact layer. The compositionand/or shape of the bottom electrode layer 850 are not particularlylimited as long as it enables the injection of carriers (e.g.,electrons) into the bottom contact layer 820. In embodiments in whichelectrons are injected into an n-type doped nitride-based semiconductorbottom contact layer 820, the bottom electrode layer 850 may include,consist essentially of, or consist of one or more metals such as one ormore metals, e.g., Ti, Al, Au, Pt, Ni, and/or V, alloys or mixtures oftwo or more of these metals, or oxide-based and/or transparent electrodematerials such as indium tin oxide (ITO). In embodiments in which holesare injected into a p-type doped nitride-based semiconductor bottomcontact layer 820, the bottom electrode layer 850 may include, consistessentially of, or consist of one or more metals having large workfunctions, e.g., Ni, Au, Pt, Ag, Rh, and/or Pd, alloys or mixtures oftwo or more of these metals, or oxide-based and/or transparent electrodematerials such as indium tin oxide (ITO). Bottom electrode layers 850 inaccordance with embodiments of the invention are not limited to thesematerials. The thickness of the bottom electrode layer 850 may be, forexample, between approximately 10 nm and approximately 100 nm, orbetween approximately 10 nm and approximately 50 nm, or betweenapproximately 10 nm and approximately 30 nm, or between approximately 25nm and approximately 40 nm.

As mentioned above, embodiments of the present invention feature agraded layer 835 disposed between the cap layer 840 and theelectron-blocking layer 830 (or the MQW layer 815 in embodiments inwhich the electron-blocking layer 830 is omitted). The graded layer 835typically includes, consists essentially of, or consists of a nitridesemiconductor, e.g., a mixture or alloy of Ga, In, and/or Al with N. Thecompositional gradient within graded layer 835 may be substantiallycontinuous or stepped, and the grading rate within the graded layer 835may be substantially constant or may change one or more times within thethickness of graded layer 835. The graded layer 835 may be undoped. Inother embodiments, the graded layer 835 is doped n-type or p-type withone or more dopants, e.g., C, H, F, O, Mg, Be, Zn, and/or Si. Thethickness of the graded layer 835 may be, for example, betweenapproximately 5 nm and approximately 100 nm, between approximately 10 nmand approximately 50 nm, or between approximately 20 nm andapproximately 40 nm. In various embodiments, the epitaxial growthprocess utilized to form the various layers of the device structure 800may be temporarily halted between growth of the graded layer 835 and theunderlying layer and/or the overlying layer. In various embodiments, thegraded layer 835 is pseudomorphically strained to one or more of theunderlying layers and/or to the substrate 805.

In various embodiments of the invention, one or more (or even all) ofthe layers of device structure 800 formed over substrate 805 may bepseudomorphically strained, similar to device layers described in U.S.Pat. No. 9,437,430, filed on Jan. 25, 2008, U.S. Pat. No. 8,080,833,filed on Apr. 21, 2010, and U.S. Pat. No. 9,299,880, filed on Mar. 13,2014, the entire disclosure of each of which is incorporated byreference herein. Thus, as detailed therein, in various embodiments, oneor more of the layers of device structure 800 may be pseudomorphic andmay have a thickness greater than its predicted (e.g., via theMaxwell-Blakeslee theory) critical thickness. Moreover, the collectivelayer structure of device structure 800 may have a total thicknessgreater than the predicted critical thickness for the layers consideredcollectively (i.e., for a multiple-layer structure, the entire structurehas a predicted critical thickness even when each individual layer wouldbe less than a predicted critical thickness thereof considered inisolation). In other embodiments, one or more layers of device structure800 are pseudomorphically strained and cap layer 840 is partially orsubstantially fully relaxed. For example, the lattice mismatch betweencap layer 840 and substrate 805 and/or MQW layer 835 may be greater thanapproximately 1%, greater than approximately 2%, or even greater thanapproximately 3%. In an exemplary embodiment, cap layer 840 includes,consists essentially of, or consists of undoped or doped GaN, substrate805 includes, consists essentially of, or consists of doped or undopedsingle-crystalline AlN, and MQW layer 825 includes, consists essentiallyof, or consists of multiple Al_(0.55)Ga_(0.45)N quantum wellsinterleaved with Al_(0.75)Ga_(0.25)N barrier layers, and cap layer 840is lattice mismatched by approximately 2.4%. Cap layer 840 may besubstantially relaxed, i.e., may have a lattice parameter approximatelyequal to its theoretical unstrained lattice constant. A partially orsubstantially relaxed cap layer 840 may contain strain-relievingdislocations having segments threading to the surface of cap layer 840(such dislocations may be termed “threading dislocations”). Thethreading dislocation density of a relaxed cap layer 840 may be largerthan that of substrate 805 and/or layers underlying cap layer 840 by,e.g., one, two, or three orders of magnitude, or even more.

In accordance with embodiments of the present invention, application oflight (e.g., laser light) and/or heat may be utilized to separate all ora portion of the substrate 805 from the rest of device structure 800, asdetailed in the '031 patent application. Before such substrateseparation, the device structure 800 may be attached to a handle wafer(not shown) by, for example, wafer bonding or an adhesive material. Invarious embodiments, the device structure 800 may be wafer bonded to thehandle wafer via use of an intermediate material such as, for example,photoresist (e.g., SU-8), glass frit, an organic material such asbenzocyclobutene (BCB), etc. Wafer bonding techniques, includingreversible ones (i.e., techniques in which the handle wafer isstraightforwardly removed after bonding and processing) are known tothose of skill in the art and may be accomplished without undueexperimentation. The handle wafer may be at least substantiallytransparent to light emitted by the active structure 815 and/or to lightutilized to separate substrate 805 (e.g., via absorption within releaselayer 810). The handle wafer may include, consist essentially of, orconsist of, for example, one or more semiconductor materials, sapphire,quartz, etc. For substrate separation, heat and/or light having awavelength corresponding to an absorption band within release layer 810(e.g., approximately 310 nm for oxygen-doped AlN) may be emitted intothe device structure 800 (e.g., from below the substrate 805 and/or fromone or more sides of the bonded structure). (As utilized herein, awavelength “corresponding to” an absorption band is sufficiently closeto the absorption band such that an amount of the light sufficient toeffect at least partial release of an underlying substrate and/or layeris absorbable within the layer having the absorption band.) In variousembodiments, the light is primarily composed of or contains a wavelengththat is within ±20 nm, within ±10 nm, within ±5 nm, within ±2 nm, orwithin ±1 nm of the wavelength of an absorption band within the releaselayer 810. In various embodiments, the release layer 810 may have morethan one absorption band (due to, e.g., introduction of two or moredifferent dopants), and the light may be primarily composed of orcontain one or more wavelengths that are within ±20 nm, within ±10 nm,within ±5 nm, within ±2 nm, or within ±1 nm of the wavelength of one ormore of the absorption bands within the release layer 810. Absorption ofthe light and/or heat within the release layer 810 results in localheating within the release layer 810, which may be magnified for releaselayers 810 having lower thermal conductivity. The local heating resultsin crack formation and subsequent fracture within the release layer 810and/or at the interface between release layer 810 and substrate 805,thereby removing the substrate 805 (or at least a portion thereof) fromdevice structure 800. In various embodiments of the invention, the lightmay be applied at a fluence ranging from, for example, approximately 500mJ/cm² to approximately 1000 mJ/cm². In various embodiments of theinvention, the light may be applied as one or more pulses. Such pulsesmay have durations ranging from, for example, approximately 10 ms toapproximately 100 ms.

In various embodiments, at least a portion of the release layer 810remains attached to the device structure 800 upon removal of thesubstrate 805. After removal of the substrate 805, any remaining portionof the release layer 810 may be removed (e.g., by selective etching orgrinding and/or polishing). A metallic contact may be formed in contactwith the bottom contact layer 820, and the device structure 800 may beutilized to emit light without absorption thereof by substrate 805. Themetallic contact may be formed on the “bottom” surface of the bottomcontact layer 820 (i.e., the surface of the bottom contact layer 820opposite the top contact layer), or a portion of the structure may beetched away so that the metallic contact may be formed on a thuslyrevealed “top” surface of the bottom contact layer (i.e., the surface ofthe bottom contact layer 820 opposite the prior location of substrate805). In various embodiments, any handle wafer used in thesubstrate-separation process is removed from the device structure 800,while in other embodiments, the handle wafer remains attached to thedevice structure 800.

As mentioned, after removal of all or a portion of the substrate 805,electrical contacts may be made to the bottom contact layer 820 and thecap layer 840 so that power may be applied to the device structure 800,resulting in light emission therefrom. FIGS. 9A and 9B depict differentdevice structures 900 in accordance with various embodiments, in which abottom electrode layer 910 is formed below the newly exposed bottomcontact layer 820 after removal of the substrate 805 (FIG. 9A) and ontop of a portion of bottom contact layer 820 after removal of thesubstrate 805 and masking and removal of a portion of the activestructure 815 (FIG. 9B). The composition and/or shape of the bottomelectrode layer 910 are not particularly limited as long as it enablesthe injection of carriers (e.g., electrons) into the bottom contactlayer 820. In embodiments in which electrons are injected into an n-typedoped nitride-based semiconductor bottom contact layer 820, the bottomelectrode layer 910 may include, consist essentially of, or consist ofone or more metals such as one or more metals, e.g., Ti, Al, Au, Pt, Ni,and/or V, alloys or mixtures of two or more of these metals, oroxide-based and/or transparent electrode materials such as indium tinoxide (ITO). In embodiments in which holes are injected into a p-typedoped nitride-based semiconductor bottom contact layer 820, the bottomelectrode layer 910 may include, consist essentially of, or consist ofone or more metals having large work functions, e.g., Ni, Au, Pt, Ag,Rh, and/or Pd, alloys or mixtures of two or more of these metals, oroxide-based and/or transparent electrode materials such as indium tinoxide (ITO). Bottom electrode layers 910 in accordance with embodimentsof the invention are not limited to these materials. The thickness ofthe bottom electrode layer 910 may be, for example, betweenapproximately 10 nm and approximately 100 nm, or between approximately10 nm and approximately 50 nm, or between approximately 10 nm andapproximately 30 nm, or between approximately 25 nm and approximately 40nm.

After formation of the electrodes 845 and/or 910, the resultinglight-emitting device may be electrically connected to a package, forexample as detailed in U.S. Pat. No. 9,293,670, filed on Apr. 6, 2015(the '670 patent), the entire disclosure of which is incorporated byreference herein. A lens may also be positioned on the device totransmit (and, in various embodiments, shape) the light emitted by thedevice. For example, a rigid lens may be disposed over the device asdescribed in the '670 patent or in U.S. Pat. No. 8,962,359, filed onJul. 19, 2012, or in U.S. Pat. No. 9,935,247, filed on Jul. 23, 2015,the entire disclosure of each of which is incorporated by referenceherein. After packaging, any handle wafer remaining on the activestructure 815 may be removed.

In accordance with embodiments of the invention, other techniques forpartial or complete substrate removal may be utilized. For example,etching techniques, such as electrochemical etching techniques describedin U.S. patent application Ser. No. 16/161,320, filed on Oct. 16, 2018,the entire disclosure of which is incorporated by reference herein, maybe utilized.

The growth of bulk single crystals has been described herein primarilyas being implemented by what is commonly referred to as a “sublimation”or “sublimation-recondensation” technique wherein the source vapor isproduced at least in part when, for production of AlN, crystallinesolids of AlN or other solids or liquids containing AlN, Al or N sublimepreferentially. However, the source vapor may be achieved in whole or inpart by the injection of source gases or the like techniques that somewould refer to as “high-temperature CVD.” Also, other terms aresometimes used to describe these and techniques that are used to growbulk single AlN crystals in accordance with embodiments of theinvention. Therefore, the terms “depositing,” “growing,” “depositingvapor species,” and like terms are used herein to generally cover thosetechniques by which the crystal may be grown pursuant to embodiments ofthis invention.

The terms and expressions employed herein are used as terms ofdescription and not of limitation, and there is no intention, in the useof such terms and expressions, of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed.

What is claimed is: 1.-73. (canceled)
 74. A single-crystal AlN substratehaving an ultraviolet (UV) transparency metric ranging fromapproximately 20 cm³ to approximately 5000 cm³ at a wavelength ofinterest of 265 nm, the UV transparency metric being defined in cm³ as:$\frac{d}{10 \times FWHM \times a^{2}}$ wherein d is a diameter of theAlN substrate in mm, FWHM is a full-width at half-maximum of an x-raydiffraction curve of the AlN substrate in radians, and a is anabsorption coefficient of the AlN substrate at the wavelength ofinterest.
 75. The AlN substrate of claim 74, wherein the diameter of theAlN substrate is at least approximately 50 mm.
 76. The AlN substrate ofclaim 74, wherein a diameter of the AlN substrate is no greater thanapproximately 150 mm.
 77. The AlN substrate of claim 74, furthercomprising a light-emitting device disposed thereover, thelight-emitting device (i) comprising a light-emitting diode or a laser,and (ii) being configured to emit ultraviolet light.
 78. The AlNsubstrate of claim 74, wherein (i) a density of threading edgedislocations in the AlN substrate is less than 5×10³ cm⁻², and (ii) adensity of threading screw dislocations in the AlN substrate is lessthan 10 cm⁻².
 79. The AlN substrate of claim 74, wherein (i) a siliconconcentration of the AlN substrate is less than 1×10¹⁷ cm⁻³, (ii) anoxygen concentration of the AlN substrate is less than 1×10¹⁷ cm⁻³, and(iii) a carbon concentration of the AlN substrate is less than 3×10¹⁷cm⁻³.
 80. The AlN substrate of claim 74, wherein a thickness of thesubstrate is 100 μm or greater.
 81. The AlN substrate of claim 74,further comprising an epitaxial semiconductor layer disposed thereover.82. A single-crystalline AlN boule having a length of approximately 15mm or larger, wherein a diameter of at least a portion of the AlN bouleis approximately 50 mm or larger, and wherein: an ultraviolet (UV)absorption coefficient of the AlN boule is less than 60 cm⁻¹ for awavelength range of approximately 220 nm to approximately 480 nm; anoxygen concentration of the AlN boule is less than 4×10¹⁷ cm⁻³; a carbonconcentration of the AlN boule is less than 4×10¹⁷ cm⁻³; and a ratio ofthe carbon concentration of the AlN boule to the oxygen concentration ofthe AlN boule is less than 1.0.
 83. The AlN boule of claim 82, whereinthe UV absorption coefficient is less than 30 cm⁻¹ for the wavelengthrange of approximately 220 nm to approximately 480 nm.
 84. The AlN bouleof claim 82, wherein (i) the oxygen concentration of the AlN boule isless than 1×10¹⁷ cm⁻³, and (ii) the carbon concentration of the AlNboule is less than 3×10¹⁷ cm⁻³.
 85. The AlN boule of claim 82, whereinthe ratio of the carbon concentration of the AlN boule to the oxygenconcentration of the AlN boule is less than 0.5.
 86. The AlN boule ofclaim 82, wherein the length of the boule is selected from the range ofapproximately 20 mm to approximately 35 mm.
 87. The AlN boule of claim82, wherein (i) a density of threading edge dislocations in the AlNboule is less than 5×10³ cm⁻², and (ii) a density of threading screwdislocations in the AlN boule is less than 10 cm⁻².
 88. The AlN boule ofclaim 82, wherein a silicon concentration of the AlN boule is less than1×10¹⁷ cm⁻³.
 89. The AlN boule of claim 82, wherein the UV absorptioncoefficient of the AlN boule is less than 10 cm⁻¹ for a wavelength rangeof approximately 350 nm to approximately 480 nm.
 90. Asingle-crystalline AlN substrate having a diameter of approximately 50mm or larger, and wherein: an ultraviolet (UV) absorption coefficient ofthe AlN substrate is less than 60 cm⁻¹ for a wavelength range ofapproximately 220 nm to approximately 480 nm; an oxygen concentration ofthe AlN substrate is less than 4×10¹⁷ cm⁻³; a carbon concentration ofthe AlN substrate is less than 4×10¹⁷ cm⁻³; and a ratio of the carbonconcentration of the AlN substrate to the oxygen concentration of theAlN boule is less than 1.0.
 91. The AlN substrate of claim 90, wherein athickness of the substrate is 100 μm or greater.
 92. The AlN substrateof claim 90, further comprising an epitaxial semiconductor layerdisposed thereover.
 93. The AlN substrate of claim 90, wherein the UVabsorption coefficient is less than 30 cm⁻¹ for the wavelength range ofapproximately 220 nm to approximately 480 nm.
 94. The AlN substrate ofclaim 90, wherein the UV absorption coefficient of the AlN boule is lessthan 10 cm⁻¹ for a wavelength range of approximately 350 nm toapproximately 480 nm.